To make this simpler, make f(x) be y.
\(f(x)= x^{2} -81\\ y=x^{2} -81\\\)
Switch x and y to find inverse
\(y=x^{2} -81\\ x=y^{2} -81\\\)
Now, solve for y.
\(x=y^{2} -81\\ x+81=y^{2} -81+81\\ x+81=y^{2}\\ \sqrt{x+81} = \sqrt{y^{2}} \\ y =\sqrt{x+81}\)
