f(x) = 5(x +4) - 6
let y = f(x)
y = 5x +20 - 6 solving for x.
y = 5x + 14
y - 14 = 5x
5x = y - 14
x = (y - 14)/5
Recall f(x) = y. Implies x = f⁻¹(y)
x = (y - 14)/5
f⁻¹(y) = (y - 14)/5
So as well:
f⁻¹(x) = (x - 14)/5 when x = 19
f⁻¹(19) = (19 - 14)/5 = 5/5
f⁻¹(19) = 1
