Answer :
f(x) = (1/9 x) - 2
y = (1/9 x) - 2
(1/9 x) = y + 2
x = 9(y + 2) = 9y + 18
f^-1(x) = 9x + 18
y = (1/9 x) - 2
(1/9 x) = y + 2
x = 9(y + 2) = 9y + 18
f^-1(x) = 9x + 18