If f(x) = 2x + 2 is invertible, then its inverse is another function f ⁻¹(x) such that
f(f ⁻¹(x)) = 2 f ⁻¹(x) + 2 = x
Solve for f ⁻¹(x) :
2 f ⁻¹(x) + 2 = x
2 f ⁻¹(x) = x - 2
f ⁻¹(x) = (x - 2)/2 = x/2 - 1
Then when x = 4, we have f ⁻¹ (4) = 4/2 - 1 = 2 - 1 = 1.
