Hi Nijo,

This DS question is based on the concept of Quadratics, but you have to be very careful with the information that you're given.

The question asks if Y = 3. This is a YES/NO question.

Fact 1: (Y - 3)(X - 4) = 0

This means that EITHER Y = 3 OR X = 4.

If Y = 3, then the answer to the question is YES.
If X = 4, then Y could be ANYTHING and the answer to the question could be YES (If Y = 3) or NO (If Y = anything else).
Fact 1 is INSUFFICIENT

Fact 2: (X - 4) = 0

This tells us that X = 4, but tells us nothing about Y.
Fact 2 is INSUFFICIENT

Combined, we know that X = 4, but we still don't know anything about Y. We run into the same situation as in Fact 1. Y might = 3, but it might be something else.
Combined, INSUFFICIENT.

Final Answer: E

GMAT assassins aren't born, they're made,
Rich