What you did was mathematically correct but it doesn't help
isolate y.

    x² + xy + y² = 9

First get 0 on the right

x² + xy + y² - 9 = 0

Rearrange in descending powers of y:

y² + xy + x² - 9 = 0 

Think of it as a quadratic equation in y this way:

1y^2 + xy + (x²-9) = 0

Then use the quadratic formula 



where a = 1, b = x, c = (x²-9)











Edwin