2分之1x-2(x-3分之1y)+(-2分之3x+3分之1y),=1/2x-2x+2/3y-3/2x+1/3y=-3x+y=-3×(-1)+2=3+2=5
1/2x-2x+2/3y-3/2x+1/3y=-3x+y把x=-1,y=2代入上式=-3乘以-1+2=3+2=5
