Rotation relative to another objects y axis

Very new to unity so apologies if this is blindingly obvious, but i’ve had a good dig around and cant find a simple way to have an object(a) rotate around an another object(b)'s y-axis, so that when you tilt object b you also tilt the angle of rotation of object a?

Its for a hammer throw type game.

Thanks for any help.

just answered similar question… about post 9 in this thread:

http://forum.unity3d.com/threads/157411-Help-with-rotation-equations