\[x^{4} + 5x^{2} - 36 = 0\]
\[x^{2} = y \geq 0:\]
\[y^{2} + 5y - 36 = 0\]
\[y_{1} + y_{2} = - 5;\ \ y_{1} \cdot y_{2} = - 36;\]
\[y_{1} = - 9 < 0;\]
\[y_{2} = 4.\]
\[x^{2} = 4\]
\[x = \pm 2.\]
\[Ответ:x = \pm 2.\]