Find the inverse of the function.

f(x) = 5x + 6

F(x) = 5x + 6

let y = f(x)

y = 5x + 6    solving for x.

y - 6 = 5x

5x = y - 6

x = (y - 6)/5

Recall   f(x) = y.   Implies  x = f⁻¹(y)

x = (y - 6)/5

f⁻¹(y) = (y - 6)/5

So as well:

f⁻¹(x) = (x - 6)/5