GRE subject test question

Hello all,

Here is a sample GRE subject exam question.

Let p(x) be the polynomial x^3 + a*x^2 + b*x + c, where a, b, and c are real constants. if p(-3) = p(2) = 0 and p'(3) < 0, which of the following is a possible value of c?

(A) -27

(B) -18

(C) -6

(D) -3

(E) -1/2

I have figured out the solution getting -27 as the answer. My question regards a solution I came across on another forum by someone else that I am having trouble following.

Given the information available is it possible to conclude that the third root of the equation must be strictly less than -3?

Thanks!