(3x+y)/2=1得3x+y=2
(5x-y)/14=1得5x-y=14
两式相加得
8x=2+14
x=2
将x=2代入3x+y=2得
y=2-3*2=-4
所以
2x-3y+7
=2*2-3*（-4）+7
=4+12+7
=23