Answer:
D
Explanation:
You add the "columns." The equations are arranged so that like terms are stacked vertically. 3x and 7x are like terms; when you add them, you get 10x.
-y and y are like terms; when you add them, you get 0 (that doesn't appear in the sum).
-5 and 15 are like terms; add them to get 10.
The sum of the two equations is the equation 10x = 10.