When Does A Set Minus B Equal B Minus A?

[physicsfun]

Homework Statement

For what conditions on sets A and B does A-B = B-A hold?

Homework Equations

A - B = A n B^c
B - A = B n A^c

The Attempt at a Solution

I know that A = B is the answer, but I just don't know where to start...please don't give me the whole proof, I want to figure it out myself. I just need a hint or an idea of where to start.

 

[Office_Shredder]

Start with: If A=B, show A-B = B-A (this should be easy)

Then suppose A-B = B-A. If $$a \in A$$ you need to prove $$a \in B$$ and vice versa (the vice versa part comes immediately from the symmetry of the problem if you want to be suave about it)