Relax

Respuesta :

f(x) = - 3x + 21.
y = - 3x + 21.

just replace x with y and y with x:

x = - 3y +21; now isolate y :

-3y = x - 21
3y = -x + 21
y = (-x+21)/3

f⁻¹(x) = (-x+21)/3