I agree with calnhob's answer and approach. In questions like these, I never go for the obvious which is setting x = 5 and y = -2. There's always a trick, unless it's the first question in the test.
My approach. First, identify what they are testing you. In an equation like this when is it equal to 0. When either set of ( ) is equal to 0 or when x=5 and y =-2. Then using that information, you move on.
I have learned through practice, never to dive into the question. First ask yourself, what is the GMAT testing, then write down either the formula or the rule. I'm not an expert in math, but it works for me.