How to prove $b=c$ if $ab=ac$ (cancellation law in groups)? I want to prove for a group $G$, that if 
$$a\circ b =a\circ c$$ then this is true $$b=c$$
I started with $b=b\circ e$, but this didn't help me at all. 
Next I tried with this:
$$(a\circ b)\circ c=a\circ (b\circ c)$$ but I don't know/understand how to go further. How can I prove this equation?

Hint:
If you know that $4\cdot x = 4\cdot y$, how do you prove that $x=y$?
Hint 2:
Think about inverses
