Polynomial division - What is the reminder ?

f(x) = (x-3)*g(x) + 21 Easy to see that f(3) is equal to 21.

Remainder Theorem:

For a polynomial $f(x)$ , the remainder of $f(x)$ upon division by $x-c$ is $f(c)$ .

By remainder theorem , we have $f(3) = \boxed{21}$ .