According to the given question we have the following expression:
if f(x) = x^2-3x, find f(3b)
So, In order to calculate f(3b) we would have to substitue the 3b at the x as follows:
f(x) = x^2-3x
f(3b)= (3b)^2 - 3(3b)
f(3b)= 9b^2 -9b
f(3b)=9b(b-1)
Therefore, f(3b) would be 9b(b-1)