I assume the question is: what is the inverse of f(x) = 5x - 3?
Change the f(x) into a y.
y = 5x - 3
Swap the y and x.
x = 5y - 3
Solve for y.
x + 3 = 5y
(x + 3)/5 = 5y/5
x/5 + 3/5 = y
Finding the inverse of an equation is as simple as replacing the f(x) with y, swapping the y and x, and solving for y. When graphed, the resulting function is an exact mirror of the original function (it is mirrored across an invisible axis).