Answer
See below.
Work Step by Step
If we want to get a formula for the inverse function, we swap $x$ and $y$ and solve for $y$.
Hence here after swapping $x$ and $y$: $x=3y+5\\x-5=3y\\y=f^{-1}(x)=\frac{x-5}{3}$.
$f[f^{-1}(x)]=f[\frac{x-5}{3}]=3\frac{x-5}{3}+5=x-5+5=x$. Hence we verified what we had to.
