xy plane problem on gmat 2.0 exam

[fangtray]

In the xy-plane, at what two points does the graph of y = (x+a)(y+b) intersect the x axis?
1. A + b =1
2. The graph intersects the y-axis at (0,-6)

[ankita1709]

If x= 0, y=ab/(1-a)-----(1)
y=0, x= -a -------------(2)

A)a+b=1
if we replace b=1-a in (1) we get y=-a
Hence no particular solution for this equation.

B)Cuts y axis at (0,-6)
It implies ab/(1-a)=-6
ab= -6(1-a)-----(3)
There are two variables and one equation. Hence no solution

But if we combine both of these
replace b=1-a in (3) we get a = -6

which means (C)

[GMATGuruNY]

The question should read:

In the xy-plane, at what two points does the graph of y = (x+a)(x+b) intersect the x axis?

I posted an explanation here:

https://www.beatthegmat.com/coordinate-g ... 83293.html

[sam2304]

In the xy-plane, at what two points does the graph of y = (x+a)(x+b) intersect the x axis?

y = x^2 + (a+b)x + ab => This is the equation of the curve. Solving for x will the values at which the curve intersects the x axis. Or this is the question rephrased. What are the values of x ?

1. A + b =1

We don't know the value of product ab in the equation - INSUFF

2. The graph intersects the y-axis at (0,-6)

Substitute the point in the eqn, x = 0 and y = -6, we get ab = -6. We don't the value of (a+b) INSUFF

Together we can find the value of x.

IMO C.