(ax^2+bx+1)(2x^2-3x-1)
=2ax^4+(-3a+2b)x^3+(-a-3b+2)x^2+(-b-3)x-1
不含x的3次项和x项则这两项系数为0
所以-3a+2b=0 (1)
-b-3=0 (2)
所以b=-3,a=2b/3=-2