Answer
$f^{-1}(x)=\sqrt{x+1},x\geq -1$
Work Step by Step
$f(x)=x^2-1,$ for $x\geq0$
$y= x^2-1$
a.
$x=y^2-1$
$y=\sqrt{x+1}$
$f^{-1}(x)=\sqrt{x+1},x\geq -1$
b. $f(f^{-1}(x))=(\sqrt {x+1})^2-1=x+1-1=x$
and
$f^{-1}(f(x))=(\sqrt {x^2-1+1})=x$