I assume by f¹, you actually mean f⁻¹ as in the inverse of f. I also assume you are asked to find f(x) (as in the inverse of f⁻¹) and f⁻¹(4).
Given that
with x ≠ 1, we can find f⁻¹(x) by replacing x + 2 with x :
Then when x = 4, we have
Of course, we also could have just substituted x = 2 into the definition of f⁻¹(x + 2) :
To find f(x), we fall back to the definition of an inverse function:
Then by definition of f⁻¹, we have
Solve for f :