Speed and angle relativity theory transformations

[Zubilaemo]

Homework Statement

http://img338.imageshack.us/img338/9682/spaceships.gif

V1, V2, U, alpha are given.

The red spaceship is moving left in Velocity V1 (in the lab system) and a ball is thrown in angle alpha and speed U(in the red spaceship system).

What is Alpha in the Blue system?

Homework Equations

lorentz transformation of speed and location

The Attempt at a Solution

I am quite clueless as to which speeds I should transform to what, and would appreciate a lending hand. thanks!

 

[Cyosis]

Resolve the x and y components of the U vector in both frames. Then transform those separately.