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