Given a function f(x), its inverse function is written as f-1(x).
An inverse function is one which reverses the job of the original function, for instance if
f(x) = 2x + 3
f-1(x) = (x-3) / 2
f(3) = (2 * 3) + 3 = 9
f-1(9) = (9 - 3) / 2 = 3
Finding an inverse function for any given function is a process of 'undoing' each operation of the original function in reverse order. For instance, f(x) above multiplied by 2 and then added 3. The inverse undid this the other way around - ot subtracted 3 then divided by 2.