(2x+ax-y+6)-(2bx^2-3x+5y-1)
=-2bx^2+(a+5)x-6y+7
因为值与字母x无关
所以
-2b=0
a+5=0
解得a=-5 b=0
3(a^2-2ab-b^2)-(4a^2+ab+b^2)
=-a^2-7ab-4b^2
=-25+0-0
=-25