posted by ALLI .
use the given zero to find the remaining zeros of the function
if 5 is a zero, then x-5 is a factor
So use either synthetic or long algebraic division to show that
x^4-15x^3+60x^2+70x-816 = (x-5)(....)
Hey, I just tried that and 5 is not a zero of your function, that is, x-5 does not divide evenly.