Looking at these two equations, you need to cancel out the variables with powers and the z. This can be simply done by adding them.
9y-2y^2+3z^2=867
+
9x+2y^2-3z^2=132
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9y+9x=999
Factor the 9 out of the left side.
9(x+y)=999
x+y=111
The answer is C (111).