A=2x^2+3ax-2x-1
B=-x^2+ax-1
3A+6B=3*(2x^2+3ax-2x-1)+6*(-x^2+ax-1)=(15a-6)x-9
3A+6B与x无关
15a-6=0,a=0.4
多项式=2x^2-5xy+6y^2-(7y^2+4xy+4x^2)=-2x^2-9xy-y^2
原式=2x^2-5xy+6y^2+(-2x^2-9xy-y^2)=5y^2-14xy
A=2x^2+3ax-2x-1
B=-x^2+ax-1
3A+6B=3*(2x^2+3ax-2x-1)+6*(-x^2+ax-1)=(15a-6)x-9
3A+6B与x无关
15a-6=0,a=0.4
多项式=2x^2-5xy+6y^2-(7y^2+4xy+4x^2)=-2x^2-9xy-y^2
原式=2x^2-5xy+6y^2+(-2x^2-9xy-y^2)=5y^2-14xy