\[x^{4} + 5x^{2} - 6 = 0\]
\[Пусть\ x^{2} = a:\]
\[a^{2} + 5a - 6 = 0\]
\[a_{1} + a_{2} = - 5;\ \ \ a_{1} \cdot a_{2} = - 6\]
\[a_{1} = - 6;\ \ \ a_{2} = 1.\]
\[Подставим:\]
\[1)\ x^{2} = - 6\]
\[нет\ корней.\]
\[2)\ x^{2} = 1\]
\[x = \pm 1.\]
\[Ответ:x = \pm 1.\]