（x^2+px+q)(x^2-3x+q)
=x^4+(p-3)x^2+(q-3p+q)x^2+(pq-3q)x+q^2
不含x^2和x^3项则这两项系数是0
所以p-3=0,q-3p+q=0
p=3,q=3p/2=9/2