Find the moment of inertia of a uniform solid circular cone of mass M, height h and base
radius a about its axis, and also about a perpendicular axis through its apex.
I'll try to get back later with more details, but the general approach is going to be to take the integral over the moment of inertia of a thin disk going from the apex to the base. Once you have that you should be able to apply the perpendicular axis theorem.
-Dan