A simple physics problem [frame of reference]

[caspernorth]

A large heavy box is sliding without friction down a smooth plane of inclination Ɵ. From a point P on the bottom of the box, a particle is projected inside the box. The initial speed of the particle with respect to the box is 'u' and the direction of projection makes an angle α with the bottom as shown in the figure.
______________________________________…
a : Find the distance along the bottom of the box between the point of projection P and the point Q where the particle lands [assume that the particle does not hit any other surface of the box. Neglect the air resistance]

b: If the horizontal displacement of the particle as seen by an observer on the ground is zero, find the speed of the box with respect to the ground at the instant when the particle was projected.
______________________________________…
http://imageshack.us/photo/my-images/834/uploadv.jpg/
______________________________________…
Answers :
a : u^2sin2α / g cosƟ
b : ucos(Ɵ + α ) / cosƟ

I could not reach answer b, is alpha + theta = 90 degree a valid assumption in this case?
this is not a homework question, I just saw this over the internet

 

[Dale]

You will probably get better responses if you post this in the homework section. Even though it is not homework it is certainly homework-style.