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