Hi jlgdr,
Since it's been over a year since you posted this question, I'm not sure if you're still here, but here's some additional perspective on how to solve it (and questions like it).
Many of the Quant questions that you'll see on the GMAT are going to be based on patterns, even if you don't immediately recognize what the patterns are (most of the Verbal questions will be based on patterns too). As such, if you're not sure how to approach a question, you should do some basic work on the pad and try to discover if a pattern actually exists.
Here, we're asked if X^2 = Y? That seems like a simple enough question.
Fact 1:\sqrt{Y} = -X
While this might look scary, Bunuel has shown how to deal with this mathematically. If you don't immediately recognize that approach, you can still "play around" with this equation...
IF...
Y = 1, then what would X equal? 1 = -X, so X = -1
With these values, we have (-1)^2 = 1, so the answer to the question is YES.
IF...
Y = 4, then what would X equal? 2 = -X, so X = -2
With these values, we have (-2)^2 = 4, so the answer to the question is YES.
IF...
Y = 9, then what would X equal? 3 = -X, so X = -3
With these values, we have (-3)^2 = 9, so the answer to the question is YES.
It seems that there IS a pattern here and the answer to the question is ALWAYS YES.
Fact 1 is SUFFICIENT.
By comparison, Fact 2 is fairly straight-forward...
Fact 2: X < Y
We can use ANY of the work that we did in Fact 1 and get a quick YES answer
If... X = 1 and Y = 2, then the answer to the question is NO.
Fact 2 is INSUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich