Relativistic Collision: Find Velocity & Composite Body

[C.E]

1.To an observer two bodies of equal rest mass collide with velocities of 0.8c in different directions and cohere to produce a stationary composite body, to a second observer body A is at rest before the collision.

a. Find the velocity of body B as measured by the second observer before the collision.

b.What is the velocity of the composite body as measured by the second oberver?

3. my answer to part a:
-1.6c/1+0.64 =-0.98c

for part b. Could somebody please show me how to do this type of question? I am really stuck on this part .

 

[NateD]

The velocity of the composite body is equal to the sum of the velocities (relative to the second observer) of the two bodies before the collision. Since body A is at rest with respect to the second observer, the velocity of the composite body is equal to the velocity of body B before the collision: -0.98c.