Answer
$\frac{x-b}{m}=f^{-1}(x)$
Work Step by Step
To find the inverse function we must "swap" $f(x)$ and $x$ and then rearrange the function so that $f^{-1}(x)$ is "alone" on one side of the equation. Hence here: $x=mf^{-1}(x)+b\\x-b=mf^{-1}(x)\\\frac{x-b}{m}=f^{-1}(x)$
