Answer
See below.
Work Step by Step
In a one-to-one function, for every $y=f(x)$there is a unique $x$. If it is not, then we cannot get a unique value for the inverse function (because there are more $x$s belonging to a $y$, whereas in a function, it must be unique). E.g. in the case of $x^2$, $f^{-1}(9)$ could be both $3$ and $-3$ , but it can only be one of them. However, there is no rule for us to decide which one of them to choose.
