Question

if f(x)=5x, what is f-1(x)

Answer 1

f=5 and x stands for itself so it should be 5-1(x) which equals 4x

Answer 2

You can say f(x) = 5x is the same as saying y = 5x  to find f^-1 you need to 1st switch the x and y and solve for y

x = 5y
y = $\frac{1}{5} x$ = $f^{-1} (x)$