Hi Vinhaha,
Since Brent has already explained the error in your work, I won't rehash any of that here.
This DS question asks us for the value of X without giving us any 'starting' information to work with.
1) 3X + 2Y = 6
In this Fact, we have to deal with another variable. By doing a bit of algebra, we can isolate the value of X...
3X = -2Y + 6
X = -2Y/3 + 2
The value of X will clearly be based on the value of Y. Since Y can be ANY value, X can be more than one value.
Fact 1 is INSUFFICIENT
2) 4Y = 12 - 6X
With this fact, we're dealing with a similar situation to the one that we dealt with in Fact 1 (above). With a bit of algebra we have...
6X = -4Y + 12
3X = -2Y + 6
At this point, you should notice that this is the SAME equation as the one we had in Fact 1, so we don't have to do any more work here (we already know the end result will be insufficient).
Fact 2 is INSUFFICIENT
Combined, we do NOT have two unique equations - we have the same equation twice. That equation was NOT enough to get us the value of X.
Combined, INSUFFICIENT
Final Answer: E
As an aside, ANY time both Facts provide the exact SAME information, the correct answer will be either D or E.
GMAT assassins aren't born, they're made,
Rich
