Help on basic set theory question. Prove or Disprove: For every two sets $A$ and $B$, $(A\cup B)-B=A$. 
I believed it was true, so first I showed that $(A\cup B)-B$ is a subset of $A$. My question is how do I prove that $A$ is a subset of $(A\cup B)-B$?
What I have first is what follows:
Suppose there exists an arbitrary element $x$ in $A$.
If $x$ is in $A$, then $x$ is not in $B$
From here, I'm stuck.

HINT: You are implicitly assuming that $A\cap B=\varnothing$.
