\[\boxed{\text{873\ (873).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[при\ x = - y:\ \]
\[- 7x + 8 = - x\ \]
\[- 6x = - 8\ \]
\[x = \frac{8}{6} = 1\frac{1}{3}\]
\[\left( 1\frac{1}{3};\ - 1\frac{1}{3} \right)\text{.\ }\]