\[\boxed{\text{921\ (921).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[1)\ x = 1;\ y = 2:\ \ \]
\[2x + y = 4\]
\[2)\ x = - 3;\ \ y = 5:\ \ \]
\[xy = - 15\]
\[3)\ x = 10;\ \ y = 0:\ \ \]
\[x + y = 10\]