(x^2+ax+b)(x^2-3x+b)
=x^4-3x^3+bx^2+ax^3-3ax^2+abx+bx^2-3bx+b^2
=x^4+(a-3)x^3+(b-3a+b)x^2+(ab-3b)x+b^2
乘积中不含x^2,x^3项,
a-3=0
a=3
b-3a+b=0
2b=3a
2b=9
b=9/2