Реши систему уравнений: {x+y=−9
{x−y=12

9x^2-12xy+4y^2=9

x+2y=9​

(3x - 2y)^2=9

|3x - 2y| = 3

1. 3x - 2y = 3

x + 2y = 9

4x = 12

x = 3

3 + 2y = 9

y = 3

2. 3x - 2y = -3

x + 2y = 9

4x = 6

x = 3/2

3/2 + 2y = 9

y = 15/4

ответ (3, 3) (3/2, 15/4)