a) $6x^3 - 24x = 0$
$\Leftrightarrow 6x(x^2-4) = 0$
$\Leftrightarrow 6x(x-2)(x+2) = 0$
$\Leftrightarrow
\left [ \begin{matrix}
x=0 \\
x = 2
\\ x = -2
\end{matrix}\right.$
b) $2x(x-3)-4x+12=0$
$\Leftrightarrow 2x(x-3) - 4(x-3)=0$
$\Leftrightarrow (2x-4)(x-3)=0$
$\Leftrightarrow
\left [ \begin{matrix}
x = 2 \\ x = 3
\end{matrix}\right.$