\[x^{2} + 20x + 91 = 0\]
\[D_{1} = 100 - 91 = 9\]
\[x_{1} = - 10 + 3 = - 7;\]
\[x_{2} = - 10 - 3 = - 13.\]
\[Ответ:x = - 13;x = - 7.\]