$f: X \to Y$ and $g: Y \to Z$ are injective functions.(for all $x\_1x\_2∈X$ if $f(x\_1)=f(x\_2)$ then $x\_1=x\_2$)
How can I prove, that $ (g \circ f)^ {-1}=f^ { -1 }\circ g^{-1 }$? (where by $f^ { -1 }$ I mean the inverse of $f$ <http://en.wikipedia.org/wiki/Inverse_function> )