The inverse of the equation is f(x) = (x - 1)/2
In order to find the inverse of any function you need to first switch the x and f(x). Then you solve for the new f(x) and that's your inverse.
f(x) = 2x + 1 ----> Switch the f(x) and x
x = 2f(x) + 1 ----> Subtract the 1
x - 1 = 2f(x) ----> Divide by 2
(x - 1)/2 = f(x)
