-x=-y-10 is the same thing as x=y+10. because u can add x to both sides and add y and 10 to bith sides to get rid of the negative. now u can substitute x in ur first equation w ur new positive equation. u should have y+10+y=12. add ur y's and numbers. u shoukd get 2y= 2. divide both sides by 2. and your answer should be y=1. hope this helps