Answer
They are inverses.
Work Step by Step
To find the inverse of a function, switch the positions of $x$ and $f(x)/y$.
The original function is $f(x)=3x$.
Switch $f(x)$ and $x$.
$x=3(f(x))$
Divide by $3$.
You are given your inverse. $f(x)=\frac{x}{3}$ which matches up with $g(x)$.
