Let
f(x) = k(x) if x > 2
f(x) = 2 + (x - 2) + (x - 2)^2 if x <= 2.
Find the function k(x) such that f is its own inverse.
https://web2.0calc.com/questions/let-find-the-function-k-x-such-that-f-is-its-own-inverse_1
This will help you, it is extremely similar. The only difference is one of the terms in the second equation. other than that, you are set.