If y=8-x then you can plug that into the equation like 7=2-(8-x). When you distribute the equation becomes 7=2-8+x. Once you combine like terms the equation becomes 7=-6+x. Then you subtract 6 from both sides, which gives you 13=x.
To find y just put 13 where x was in the first equation. Y=8-13.
Y=-5
X=13