Answer
See proof below.
Work Step by Step
Given $I(x)=x$, we have
$f\circ I=f(I(x))=f(x)$
$I\circ f=I(f(x))=f(x)$
$f\circ f^{-1}=f(f^{-1}(x))=x=I(x)$ use fundamental property of inverse functions
$f^{-1}\circ f=f^{-1}(f(x))=f^{-1}(y)=x=I(x)$