Example LF.INV.2 : (i) Suppose $f(x) = 3x$. Verify that $g(x) =
\frac x 3$ is the inverse function for $f$.
ii)Suppose $f(x) = x +2$. Verify that $g(x) = x - 2$ is the
inverse function for $f$.

We check (i) visually both with the graph and the mapping
diagram.
Which do you find more convincing visually?