\[x^{3} + 3x^{2} - 4x - 12 = 0\]
\[x^{2} \cdot (x + 3) - 4 \cdot (x + 3) = 0\]
\[(x + 3)\left( x^{2} - 4 \right) = 0\]
\[(x + 3)(x - 2)(x + 2) = 0\]
\[x = - 3;\ \ \ \ x = 2;\ \ \ \ \ x = - 2\]
\[Ответ:x = - 3;\ x = \pm 2.\]