how to prove: $A=B$ iff $A\bigtriangleup B \subseteq C$ I am given this: $A=B$ iff $A\bigtriangleup B \subseteq C$. And $A\bigtriangleup B :=(A\setminus B)\cup(B\setminus A)$. 
I dont know how to prove this and I dont know where to start. 
please give me guidance

Hint: For an arbitrary set $C$, what is the one and only set that is the subset of every set?
So given $\,A\triangle\,B \subseteq C$, where $C$ is any arbitrary set, what does this tell you about the set $A\triangle B$?   
And what does that tell you about the relationship between $A$ and $B$?
