решение во вложении
x-y=3
x^2-xy-2y^2=7
x=3+y
(3+y)^2-(3+y)y-2y^2=7
9+y^2-3y+y^2-2y^2=7
-3y=-2
y=2/3
x=3+2/3=11/3
x=11/3
