A=2a^2+3ab-2a-1,B=-a^2+ab-1,且3A+6B的值与a无关
3A+6B=3(2a^2+3ab-2a-1)+6(-a^2+ab-1)
=6a^2+9ab-6a-3-6a^2+6ab-6
=15ab-6a-9
=(15b-6)a-9
15b-6=0
b=2/5