You can "add" like you say or you could just call it subtracting. If you subtract 5 from 5 you still get zero. Then on the right side, you would do the same. You would subtract 5 from 12 getting x, which is 7.
If you added you would still get the same answers but it is easier to say just subtract. Anything you do to the left, do to the right.
Hope this helps :)
<span>X+2y=10 ----Eq.1
</span><span>x+y=6---------Eq.2
from Eq2.
x=6-y
Subtituting the value of X in the first equation
6-y+2y=10
6+y=10
y=4
Plug y=4 in equation 2.
</span><span>x+4=6
</span>X= 2
