I'm having a tough time with this one. Here's the background:
Let $X$ and $Y$ be sets, let $f:X\rightarrow Y$ and let $A,B\subseteq X$. For this proof, we also assume that $f$ is 1-1.
I've already proven $f(A\setminus B)\supseteq f(A)\setminus f(B)$, but where should I start to prove the other way around?