Geva@MasterGMAT wrote:Question is asking at what point does y equal zero. This happens in one of two cases: when x=-a, or when x=-b. thus, in order to answer the question, you need to know the values of a and b.

Stat. (1): one equation, two unknowns - cannot find a and b.

Stat. (2): what this tells you is that when x=0, y must equal -6. Plug these into the equation to get x=0 into the equation to get -6=(0+a)(0+b), or ab=-6. Again, one equation, two unknowns - cannot find a and b.

Combined: two equations, two unknowns - you can find a and b. Note that for this particular pair (the sum and product of a and b), you will find two solutions - either a=2 and b=-3, or vice Versa: a=-3, b=2 - but in both cases you know that the x intercepts are the reverse :3 and -2. Sufficient - answer is C.

prachich1987 wrote:

Geva@MasterGMAT wrote:Question is asking at what point does y equal zero. This happens in one of two cases: when x=-a, or when x=-b. thus, in order to answer the question, you need to know the values of a and b.

Stat. (1): one equation, two unknowns - cannot find a and b.

Stat. (2): what this tells you is that when x=0, y must equal -6. Plug these into the equation to get x=0 into the equation to get -6=(0+a)(0+b), or ab=-6. Again, one equation, two unknowns - cannot find a and b.

Combined: two equations, two unknowns - you can find a and b. Note that for this particular pair (the sum and product of a and b), you will find two solutions - either a=2 and b=-3, or vice Versa: a=-3, b=2 - but in both cases you know that the x intercepts are the reverse :3 and -2. Sufficient - answer is C.

Hi Geva,

Thanks for the explanation
I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?

Anurag@Gurome wrote:

prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?

The answer is not A, it's C.
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.

prachich1987 wrote:

Anurag@Gurome wrote:

prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?

The answer is not A, it's C.
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.

It was a type error to write "A"
I actually mean to say "how can the answer be C"
The question is asking us "at what point does y = (x + a)(x + b) cross the x axis?"

But we have got two points.
Hence even after combining the two statements , we don't get a unique value as it is intersecting at both (2,0) & (-3,0)

Geva@MasterGMAT wrote:

prachich1987 wrote:

Anurag@Gurome wrote:

prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?

The answer is not A, it's C.
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.

It was a type error to write "A"
I actually mean to say "how can the answer be C"
The question is asking us "at what point does y = (x + a)(x + b) cross the x axis?"

But we have got two points.
Hence even after combining the two statements , we don't get a unique value as it is intersecting at both (2,0) & (-3,0)

I know this GMATPREP question: I believe the original question DID ask for points (plural), rather than point (singular). In any case, the former's answer is C, for reasons explained; the latter answer's E, for the reason you provide.

ankurmit wrote:y = (x + a)(x + b)

y = x^2 +x (a+b) +ab

At point line crosses X-axis, Y=0

hence x^2+x(a+b)+ ab =0

Stem 1: Gives value of a+b but not value of ab

hence not sufficient

Stem 2:. graph intersects y axis at (0, -6)

y = x^2 +x (a+b) +ab

put values in equation we get value of ab

hence not sufficient

Combining both we get value of a+b and ab both

hence C is answer

prachich1987 wrote:

Anurag@Gurome wrote:
The answer is not A, it's C.
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.

It was a type error to write "A"
I actually mean to say "how can the answer be C"
The question is asking us "at what point does y = (x + a)(x + b) cross the x axis?"

But we have got two points.
Hence even after combining the two statements , we don't get a unique value as it is intersecting at both (2,0) & (-3,0)