You haven't said what you need to "do" with it. I'm going to take a wild guess that you've been asked to 'factor' it. I don't have any step-by-step method for realizing that, and then doing it. But if you've done all the homework on multiplying twobinomials, and kind of almost understood it, then you might recognize this expression as the result AFTER two binomials got multiplied. Factoring it means to un-multiply it, and get the original two binomials. This is the product of (a-x) times (a+y).
