Question 1042709
Rational Roots Theorem tells us here that possible roots to check are plus and minus of 1,3,5,15.  Use synthetic division to check each root; remainder of 0 means, root checked is a root of the function.


ONLY ONE rational root, being {{{-5}}}.
The two others are both complex and irrational.
(What are the roots for {{{x^2-x+3}}} ?)