\[x^{3} - 81x = 0\]
\[x\left( x^{2} - 81 \right) = 0\]
\[x(x - 9)(x + 9) = 0\]
\[x = 0;\ \ x = 9;\ \ x = - 9.\]
\[Ответ:x = 0;\ \ x = \pm 9.\]