Rebecca R. answered 09/12/18
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Experienced Elementary Math, Prealgebra, Algebra 1, and Geometry Tutor
x + y = -9
x - y = 31
from 1st equation, x = -9 - y
substitute into 2nd equation:
-9 - y - y = 31
-9 -2y = 31
2y = -40
y = -20
x = -9 - y = -9 - (-20) = -9 + 20 = 11
x = 11
y = -20