（2x^2+ax-y+6 )-(2bx^2-3x+5y-1)
=2x^2+ax-y+6-2bx^2+3x-5y+1
=（2-2b)x²+(a+3)x-6y+7
因与x值无关
所以2-2b=0,即b=1
a+3=0,即a=-3
3 (a^2-2ab-b^2)-(4a^2+ab+b^2)
=3a²-6ab-3b²-4a²-ab-b²
=-a²-7ab-4b²
=-(-3)²-7(-3)(1)-4(1)²
=-9+21-4
=8