One factor would be (x+4). Then you have (x-(6i)), or (x-6i). Since it is an imaginary solution, there is a positive and a negative version. So the third factor would be (x-(-6i)) or (x+6i). Then you multiply. (x-6i)(x+6i) is x^2+36. Multiply that by (x+4) and get a final answer of x^3+4x^2+36x+144.
