We will need to make both equations equal to the same subject, in this case we will make them both equal to x:
-x-y=5
-y-5=x
x+2y=5
x=5-2y
Now because they are both equal to x, they are therefore equal to each other:
-y-5=5-2y
Now we solve for y:
y=10
Now that we have y, we can substitute it back into either original equation to get x (always use the easier one):
-x-10=5
-15=x
I hope this counts as using addition.
