Verified answer

f(x) = (x + 3)/3, so let y = (x + 3)/3

to solve for the inverse, you replace x and y's (since the inverse function effectively reflects the original function in the line y = x, which brings the point (x, y) to (y, x))

so x = (y + 3)/3

3x = y + 3

y = 3x - 3

f^-1(x) = 3x - 3 is the required inverse.