3A+6B=3(3X^2+3aX-2X-1)+6(-X^2+aX-1)=3x^2+(9a-6+6a)x-3-6=3x^2+(15a-6)x-9
因为:3A+6B的值不含x项,所以:15a-6=0
解得:a=6/15=2/5