Hello.
Firstly, let's isolate x on the equation:
[tex]\mathsf{f(x) = 3x + 3} \\
\\
\mathsf{y = 3x + 3} \\
\\
\mathsf{-3x = -y + 3} \\
\\
\mathsf{x = \dfrac{-y + 3}{3}}[/tex]
Now, we can replace 'x' for 'y' and vice-versa:
[tex]\mathsf{f^{-1}(x) = \frac{-x + 3}{3}`}[/tex]
Hope I helped.
