(i) z^9+1=(z^3+1)(z^6-z^3+1)
(ii) roots of (z^3+1) are cis(pi/3), cis(pi), cis(-pi/3)
roots of (z^9+1) are cis(pi/9), cis(pi/3), cis(5pi/9), cis(7pi/9), cis(pi), cis(-7pi/9), cis(-5pi/9), cis(-pi/3), cis(-pi/9)
so roots of (z^6-z^3+1) are cis(pi/9),cis(5pi/9), cis(7pi/9), cis(-7pi/9)...