we use equation 1 first.
-2x -y = -9
-y = -9 + 2x
y = 9 -2x
We substitute value of y in equation 2
5x - 2 (9 - 2x) = -9
5x -18 - 4x = -9
5x - 4x = -9 + 18
x = 18 -9
x = 9
We now find the value of y
-2x -y = -9
-2 (9) - y = -9
-18 - y = -9
-y = -9 + 18
y = 9 - 18
y = -9