\[\boxed{\mathbf{726\ (726).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\frac{x_{1}}{x_{2}} = \frac{7}{3},\ \ 3x_{1} = 7x_{2},\ \ \]
\[x_{1} = \frac{7x_{2}}{3}\]
\[x^{2} + 20x + a = 0\]
\[x_{1} + x_{2} = - 20\]
\[\frac{7x_{2}}{3} + x_{2} = - 20\]
\[7x_{2} + 3x_{2} = - 60\]
\[10x_{2} = - 60\]
\[x_{2} = - 6\]
\[x_{1} = - 20 - x_{2} = - 20 + 6 =\]
\[= - 14\]
\[x_{1} \cdot x_{2} = a\]
\[- 6 \cdot ( - 14) = 84\]
\[Ответ:\ x_{1} = - 14;\ \ x_{2} = - 6;\ \ \]
\[a = 84.\]