First, find the possible values of x in the given equation. x^2 - 9 = (x + 3)(x - 3), so since that expression is set equal to zero, x could be 3 or -3. Since we're told that x < 0, x must be -3.

There are two ways to identify the correct choice. You can plug -3 into each of the five choices, or you can factor each of the five choices, looking for one that includes the term (x + 3). Try the latter:

(A) = x(x - 9)
(B) = (x - 5)(x - 4)
(C) = (x - 3)(x + 1)
(D) = (x + 3)(x - 1)
(E) = (x + 5)(x + 1)

(D) is correct.