A simple factoring problem $3x^3 - x^2 -12x + 4$ I'm stuck on a simple factoring problem of 
$$3x^3 - x^2  -12x + 4$$
I keep coming up with 
$$x^2(3x - 1) -4(3x - 1) = (3x - 1) (x^2 - 4) = 3x^3 -12x - x^2 + 4 =
3x^3 - x^2 - 12x + 4$$
but it doesn't seem to be right, can anyone point out my mistake?

From $x^2(3x - 1) -4(3x - 1)$, factor out $(3x-1)$ to get $(x^2-4)(3x-1)$. 
Knowing that $a^2-b^2=(a+b)(a-b)$, we get $(x-2)(x+2)(3x-1)$.
