Given:
x^2+xy+y^2=18
x^2+y^2=12
Attempt:
(x^2+y^2)+xy=18
12+xy=18
xy=6
y^2=12-x^2
(12)+xy=18
xy=6
Attempt 2:
xy=6
x=y/6
y^2/36+(y/6)y+y^2=18
43/36y^2=18
y ≠ root(6) <- should be the answer
Edit:
Just realized you can't plug the modified equation back into its original self
I plugged y=6/x...