If it has a root at -3, we know that (x + 3) is a guaranteed root, because to a root from a factor, you simply set the expression inside the parentheses equal to zero and solve for x.
(x + 3) = 0
x = -3
We do the same thing but in vice versa to find the factor.
x = -3
x + 3
Thus, the factor is (x + 3)
