9x - 8y = -11
-9x + 3y = 21
Add the second equation
+ (-9x) + 3y = 21
-5y = 10
divide both sides by -5
y = -2
substitute y into one of the two equations in the beginning
9x - 8(-2) = -11
9x + 16 = -11
subtract 16 on both sides
9x = -27
divide both sides by 9
x = -3
The answer is (-3, -2)
