6mx²-8nxy+3x-4xy-x²+y-3
=(6m-1)x²+(-8n-4)xy+3x+y-3
∵关于x,y的多项式6mx²-8nxy+3x-4xy-x²+y-3中不含二次项
∴6m-1=0
-8n-4=0
∴m=1/6,n=-1/2
3m-4n=1/2+2=2.5