f(x) = 6(x +2) - 3
let y = f(x)
y = 6x + 12 - 3
y = 6x + 9 solving for x.
y - 9 = 6x
6x = y - 9
x = (y - 9)/6
Recall f(x) = y. Implies x = f⁻¹(y)
x = (y - 9)/6
f⁻¹(y) = (y - 9)/6
So as well:
f⁻¹(x) = (x - 9)/6 when x = 21
f⁻¹(21) = (21 - 9)/6 = 12/6 = 2
f⁻¹(21) = 2
