3a-4b=-1
ax^2+2bxy-x^2-3x-2xy+y
=(a-1)x^2+(2b-2)xy-3x+y
根据题意,“不含二次项”即
a-1=0 a=1; 2b-2=0 b=1
因此,3a-4b=3*1-4*1=-1