An astronomer sees two objects moving along the same line of sight away from each other. The first object moves away from the Earth with a velocity of 2.5×108 m/s, and the second object moves towards the Earth with a velocity of 1.8×108 m/s.
According to this astronomer how fast are the two objects moving away from each other?
V'x = (Vx - u)/(1-(u*Vx/c^2))
Vx = (V'x + u)/(1+ (u*V'x/c^2))
The Attempt at a Solution
I'm not sure how to apply the transformations here.
Should I take the the reference frames used in the equations to be on the objects and attempt to find u?
Normally In a problem like this I would take the Earth to be the first frame of reference