Answer
$f(g(x))=x$ and $g(f(x))=x$.
Work Step by Step
$f(x)=4x+9, g(x)=\frac{x-9}{4}$
$f(g(x))=4\left(\frac{x-9}{4}\right)+9=x$
and
$g(f(x))=\frac{4x+9-9}{4}=x$.
$f(g(x))=g(f(x))=x.$
Thus, $f(x)$ and $g(x)$ are Inverse of each other.