You may want to review function notation and one-to-one functions first.

The important thing about one-to-one functions is that they have
inverses. The inverse of a function is the function that undoes
it. For example, if the function is *f*(*x*) =
−3*x*, then its inverse
if *f*^{−1}(*x*) = *x* / (−3),
since dividing by −3 and multiplying by −3 undo each
other. (*f*^{−1} is just a symbol for the
inverse of *f*.) If *f*(*x*) = 2*x* + 1,
then *f*^{−1}(*x*) = (*x* − 1)
/ 2. Note that we have to switch the order; in *f*
we *first* multiply by 2 and *then* add 1, so to
undo it we have to *first* subtract the 1 and *then*
divide by the 2.

There are some functions that are their own inverses. One example
is *f*(*x*) = −*x*, since that undoes itself.
Another example is *f*(*x*) = *x*, since then
there's nothing to undo!