Find the formul for df-1/dx

Find the formula for df-1/dx

f(x)= 1/8x + 1/16

A. 8x- 1/2
B. 1/8
C. 8
D. x - 1/2

Help step solve this please!

Replace f(x) with y temporarily for convenience.
y = 1/8x + 1/16
Swap x's and y's and solve for y:
x = 1/8y + 1/16
x - 1/16 = 1/8y
8x - 1/2 = y
f^-1(x) = 8x - 1/2
df^-1/dx = 8
Answer is C.