Two functions are defined as
and
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What do I need to do?
$\displaystyle f^{-1}$ is a function inverse to f. Note that (not necessarily in your problem, but in general) for some y there may be several different x such that f(x) = y. In this case, it is not clear to which of those x the inverse function $\displaystyle f^{-1}$ should map y. Therefore, the concept of inverse function is unambiguous only if the original function is injective.