Restrict the domain of $f(x)$ to find inverse:
\begin{align} f(x) & = x^2+6x+9 = (x+3)^2 \\ g(x) & = \sqrt{x} - 3 \end{align}
Restrict the domain of $f(x)$ to find inverse:
\begin{align} f(x) & = x^2+6x+9 = (x+3)^2 \\ g(x) & = \sqrt{x} - 3 \end{align}