F( 1)=0, so a+b+c=0.

F(2)=0, so 8a+4b+2c=0. Subtract the above formula twice to get 6a+2b=0, b=-3a, and c=-a-b=2a.

When x; 2 when F(x) is greater than 0, a >；; 0, so b =-3a.

So f (x) = a (x 3-3x 2+2x)