Long way: try to divide. But we are lazy, so it's no go. Option number one: all rational numbers that could give you a division can be taken from "THE List": you build it by taking the constant term, that is 6, and all its divisors (1,2,3,6), the highest term coefficent, 3 in this case, its divisors (1,3) , and you divide every one in the first list by the second, and adding a
in front of them. Our list is:
.
Since -3 is part of THE List, it might be a divisor - that system would garantee you that, for example, x-4 would NOT be a factor.
Cheap way. if (x+3) is a factor of g(x), then g(-3) = 0. Let's replace.

Since we didn't get zero,
is not a factor of g(x)