Answer
$f^{-1}(x)=\frac{9-x}{2}$
Work Step by Step
In order to find to the inverse function of $y=f(x)$ we have to swap $x$ and $y$ $x=f(y)$ and then express $y$ as a function of $x$.
In our case $y=f(x)=9-2x$. So $x=9-2y$ or $2y=9-x.$
$$y=\frac{9-x}{2}$$
Our inverse function is
$$f^{-1}(x)=\frac{9-x}{2}$$
