a+b=0
cd=1
x=3a-3-a+2b=2a+2b-3=2(a-b)-3=-3
y=c(cd)-cd-c+1=c-1-c+1=0
原式=(4x+2y-3x+2y)/6
=(x+4y)/6
=(-3+0)/6
=-1/2