Annoying M2 Question

Hey,

This is question 5. on M2 June 2006 Edexcel A-level Maths.

A vertical cliff is 73.5m high. Two stones A and B are projected simultaneously. Stone A is pojected horizontally from the top of the cliff with speed 28m/s. Stone B is projected from the bottom of the cliff with speed 35m/s at an angle (alpha) above the horizontal. The stones move freely under gravity in the same vertical plane and collide in mid-air. By considering the horizontal motion of each stone,

a) prove that cos(alpha) = 4/5

b) find the time which elapses between the instant when the stones are projected and the instant when they collide.

I've tried for ages on part a) so any pointers would be greatly apprecitated, for part b) too maybe. I'm not asking for an answer but a nudge in the right direction would be ace.