\[9x² - 20x - 21 = 0\ \ \ \ \ \ и\ \ \ \ x_{1} = 3\ \]
\[x² - \frac{20}{9}x - \frac{7}{3} = 0\]
\[x_{1} + x_{2} = \frac{20}{9} \Longrightarrow 3 + x_{2} =\]
\[= 2\frac{2}{9} \Longrightarrow x_{2} = - \frac{7}{9}\]
\[x_{1} \cdot x_{2} = - \frac{7}{3} \Longrightarrow 3 \cdot x_{2} = - \frac{7}{3} \Longrightarrow\]
\[\Longrightarrow x_{2} = - \frac{7}{9}\]
\[Ответ:x_{2} = - \frac{7}{9}.\]