Solution:
x + y = 11, -x = -y - 9
~Isolate x in [ x + y = 11 ]
x = 11 - y
~Substitute x in the other equation
-(11 - y) = -y - 9
~Simplify
-11 + y = -y - 9
y = -y + 2
2y = 2
y = 1
~Substitute y in the first equation
x + 1 = 11
~Simplify
x = 10
Therefore, the solution to the system of equations is (10, 1)
Best of Luck!