\[ax^{2} - 5x - 2 = 0;\ \ x_{1} = 2:\]
\[4a - 5 \cdot 2 - 2 = 0\]
\[4a - 12 = 0\]
\[4a = 12\]
\[a = 3.\]
\[Получаем\ уравнение:\]
\[3x^{2} - 5x - 2 = 0\]
\[D = 25 + 24 = 49\]
\[x_{1} = \frac{5 + 7}{6} = 2;\]
\[x_{2} = \frac{5 - 7}{6} = - \frac{2}{6} = - \frac{1}{3}.\]
\[Ответ:x_{2} = - \frac{1}{3}.\]