Soln:
Let 4x + 5 = a
or, x = (a-5)/4
Then,
f(a) = 12*(a - 5)/4 + 18
= 3*(a - 5) + 18
= 3a - 15 + 18
= 3a + 3
Therefore,
f(x) = 3x + 3
Let y = f(x)
or, y = 3x + 3
or, x = (y - 3)/3
Interchanging the role of x and y, we get,
y = (x - 3)/3
f-1(x) = (x - 3)/3 Ans.
