\[Пусть\ \ \ t = x^{2} - 8:\]
\[t^{2} - 5t - 14 = 0\]
\[t_{1} + t_{2} = 5;\ \ t_{1} \cdot t_{2} = - 14\]
\[t_{1} = 7,\ \ t_{2} = - 2.\]