\[\boxed{\text{972\ (972).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[(a;\ 2a);\ \ x = a;\ \ y = 2a:\]
\[2x + 7y = 16\]
\[2a + 14a = 16\]
\[16a = 16\]
\[a = 1\]
\[Ответ:при\ a = 1.\]