f-1(x) = 8x - 24
You can get this by switching the x and f(x) and then solving for f(x)
f(x) = 3 + 1/8x ---> now switch them
x = 3 + 1/8f(x) ---> now subtract 3 from both sides
x - 3 = 1/8f(x) ---> now multiply both sides by 8
8x - 24 = f(x) ----> ANSWER
