Use the elimination method to solve the system of equations.

x+y=-3
x-y=13

So I will choose to cancel the y first because that is easiest. To do this, add both equations together.
x+y = -3
+
x - y = 13
This will give us:
2x = 10
therefore, x = 5
Now, we can plug this into the first equation to find y
5+y = -3
y = -3 - 5 
y = -8

x = 5
y = -8

Hope this helped!! :D

Isolate X from the first:
X=-3-y substitute in the second:
-3-y-y=13
-2y=16 so y=-8
Back in the first:
X-8=-3
X=5

The answer is why would a teacher ask that