X + Y = 0
X - 2Y + 9 = 0
------I don't like that top equation, so...,
X - 2Y + 9 = 0
X = 2Y - 9
------Now, put this into that top equation.
(2Y - 9) + Y = 0
3Y - 9 = 0
3Y = 9
Y = 3
======Obviously, X = - 3
======check in the other equation
- 3 - 2(3) + 9 = 0
- 3 - 6 + 9 = 0
0 = 0
------all checks and is consistant