You can find the inverse of an equation by swapping the x and y, then solving for y.
f(x) = 4x - 12 f(x) is the equivalent of y in this case, so change it to a y
y = 4x - 12 Swap the x and y variables
x = 4y - 12
Solve for y
x = 4y - 12 Add 12 to both sides
x + 12 = 4y Divide both sides by 4
+ 3 = y Switch the sides to make it easier to read
y = + 3 Change y back into f(x) to match what you were given
f(x) = + 3
