Answer
Since $f\circ g(x)=x$ and $g\circ f(x)=x$,
f and g are inverses of each other.
Work Step by Step
$\begin{array}{lllll}
f\circ g(x) & =f[g(x)] & ..... & g\circ f(x) & =g[f(x)]\\
& =3g(x)+4 & & & =\frac{f(x)-4}{3}\\
& =\frac{3(x-4)}{3}+4 & & & =\frac{3x+4-4}{3}\\
& =x-4+4 & & & =\frac{3x}{3}\\
& =x & & & =x
\end{array}$
Since $f\circ g(x)=x$ and $g\circ f(x)=x$,
f and g are inverses of each other.