make 4+y= -x as equation (1) and make y=2x-10 as equation (2, then you must find a third equation either from equation (1)or (2). If you choose to find equation (3) from (2) it will be y=2x-10 , then substitute equation (3) from (1) because you can't choose (2) , then it will be 4+(2x-10)= -x