Answer
$7$
Work Step by Step
By the definition of an inverse one-to-one function, if $f(x)=y$, then we have $f^{-1} (y)=x$.
Therefore, when $f(7)=13 $ then its inverse can be expressed as: $f^{-1} (13) =7$
In other words, when finding the inverse of a function, the $x$ and $y$ values get switched.
