\[Пусть\ \ t = x^{2} - 3x:\]
\[t^{2} - 8t - 20 = 0\]
\[t_{1} + t_{2} = 8;\ \ t_{1} \cdot t_{2} = - 20\]
\[t_{1} = 10,\ \ t_{2} = - 2\]