(1)-(2)×2得
-7x=2-a
∴7x=a-2 (3)
(1)×5-(2)×3得
7y=2a+10 (4)
(3)+(4)得
7x+7y=3a+8
7(x+y)=3a+8
∵x+y=2
∴3a+8=14
∴a=2