f-1(x) = (x-7)/2
In order to find the inverse of a function, reverse the equation
Let's say f-1(x) = y (to keep it as a go to, but I use it to make it simpler to see the steps )
y = 2x+ 7
y - 7 = 2x
(y - 7) / 2 = x
=> f-1(x) = (x - 7) / 2
NB:
The idea is to "revert" the arithmetic of the equation