f(x) = -5x - 15
Let y = f(x)
y = -5x - 15
Switch the x and y places, and solve for y:
x = -5y - 15
-5y = x + 15
y = (x + 15)/-5
y = -(x/5) - 3
So the inverse of : f(x) = -5x - 15 is
-x/5 - 3
