To add these equations you just have to combine like terms on the same side of each equation, like this:
As you can see, we got 2x + 7y -2x -8y on the left, by combining like terms and performing the subtraction on the right side, we get:
2x-2x+7y-8y=2
0-1y=2
-y=2
Then, after adding these equations, we get -y = 2
