Let A and B be sets and X and Y be subsets of A. Let f : A --> B be

a function. Prove that f(X) - f(Y) is a subset of f(X - Y).

I'm not very comfortable with sets to begin with, but now that I've got functions thrown in and a really bad textbook, I'm lost. I've only gotten to "Let a be a subset of f(X) - f(Y). Then a is in f(X) and a is not in f(Y)." but now I'm not sure where to go next. Any help at all would be greatly appreciated. Thanks. :o)