**What have you tried and where are you stuck?**

Also, Is that supposed to be:

> f(x) = x^2 - 6x

or is the formatting in the post correct?

If the function *is* meant to be a quadratic then the inverse (and whether or not one exists) will depend on the intended domain and codomain. For example, if f is defined as a function from the Reals to the Reals, it has no inverse.