To solve the inverse, one way to do it is simply rearranging or switching the X and y coordinate values, then solve for the new y coordinate.
Y = 5x + 4
X = 5y + 4
X - 4 = 5y + 4 - 4
5y = X - 4
5y/5 = X - 4/5
Y = (X - 4)/5
The inverse of the original function is y = (X - 4)/5.