There are several ways to do this.

I think the one common in College Algebra class is:

For y = f(x), you have

To find interchange all x's & y's, i.e., wherever there was an x place a y & wherever a y (usually just the one) place a x. Then solve for y. What you then have for y is the same as what is meant by . In your case solve for y and that's the same as the inverse function.