Answer
$7$
Work Step by Step
If for a function $f(x)=y$ then for the inverse function $f^{-1}(y)=x.$
Since $f$ is one-to-one then for each $x$ there is a unique $f(x)$ and all the $f(x)$ values are distinct.
Hence if $f(x)=y$, then $f^{−1}(y)=x$.
With $f(7)=13$, then $f^{−1}(13)=7.$
