Observe that all terms have x as a factor, so pull that out first, then you are left with a quadratic...to factor this you need two factors of -12 whose sum is 1. Can you finish?
Factored form should be (x-3)(x+4) = x^2 +4x -3x -12 = x^2 +x -12 = x(x^2 +x -12) = x^3 +x^2 -12x
So the factored form should be just: x(x^2 +x -12) or not???
Completely factored is $\displaystyle x(x+4)(x-3)~.$