f(x)=ax^2+bx+c
A(-1,0),
0=a-b+c (1)
B(3,0)
0=9a+3b+c (2)
C(1,-8)
-8=a+b+c (3)
(2)-(1) 0=2a+b
(2)-(3) 4=4a+b
a=2 b=-4 c=-6
f(x)=2x^2-4x-6
=2(x^2-2x-3)能化简吗，化简带进去为什么就不对了