The search on primitive roots got me some good info, but I can not seem to bring it together to answer this problem.

Show that if (a,15) = 1, then a^(phi(15)/2) congruent 1(mod15) and hence 15 has no primitive roots. [Hint: Examine the congruence (mod 3) and (mod 5).]

I am unsure of how to integrate the hint into the problem.