Do you remember the definition of inverse functions?
$f(x)$ and $g(x)$ are inverses if $f(g(x))=g(f(x)) = x$...
Well, $f(f ^{−1}(x)) = e^{\ln x }$ ______

Refer to the hint in part (a).

Refer to the hint in part (a).

Let $3x = U$.