Answer
$f(g(x))=x$ and $g(f(x))=x$.
Work Step by Step
$f(x)=3x+8, g(x)=\frac{x-8}{3}$
$f(g(x))=3\left(\frac{x-3}{3}\right)+8=x$
and
$g(f(x))=\frac{3x+8-8}{3}=x$.
$f(g(x))=g(f(x))=x.$
Thus, $f(x)$ and $g(x)$ are Inverse of each other.