x^3+ax-y+b-( bx^3-3x+6y-3)
=(1-b)x³+(a+3)x+5y+b+3
∵值与x的值无关
∴1-b=0
a+3=0
∴a=-3
b=1
3（a^2-2ab-b^2)-4a^2-2ab+6b^2÷2
=3a²-6ab-3b²-4a²-2ab+3b²
=-a²-8ab
=-9+24
=15