(x^2+ax+b)(x^2-2x-3)
=x^4+(a-2)x^3+(b-2a-3)x^2+(-2b-3a)x-3b
不含x^3与x^2项则这两项系数为0
所以a-2=0
b-2a-3=0
a=2
b=2a+3=7