GRE Algebra - Positive/Negative

Aside from solving an algebraic equation the regular way, there are many ways in which you can eliminate several possible choices; one of which is by noticing that some solutions to algebra questions must be positive, while others must be negative. For example, consider the following question:

Billy has x apples and y oranges in a basket. If Billy has 9 oranges, and the number of apples squared equals the number of oranges, then how many apples does Billy have?

We are told that y = 9, so we set up the following equation: x2 = 9. Now we know that both 3 and (-3) squared equal 9, but it's obvious that Billy can't have -3 apples, because you can't have a negative number of apples, so you could eliminate all answer choices that are negative.