Why f(8)=0?Cs we use a Thm, It is unknow for most people here.

Thm can be verified by my method. Young man, I'm here for fun and health. I like your self-confidence and gusto....In fact,the Q is not good in strict Math, we should use...repeat root 5, Why?...,if not It is against Algebraic Fundamental Thm.

At last,I give the general answer...for 15less:

Only change 2 to d,5 to c,0 to t ,f(0) to f(t) in originnal Q.

f(t)=-(d/(d-c)^2)(t-d)(t-2c+d)   if d<c

f(t)=-((2c-d)/(d-c)^2)(t-d)(t-2c+d) if d>c.