A rocket approaches a mirror on the ground at a speed of 0.90c. The distance D between the front of the rocket and the mirror is 2.4 * 10^12 m, as measured by ground observers, when a light signal is sent from the front of teh rocket. When is the reflected signal recieved by the rocket as measured by a) the observers on the ground b) the observers on the rocket? I tried for a) working out the time interval using the formula and substituting the values known of speed of rocket and distance of rocket from mirror but I ended up with a huge value. The actual answer is given as 8.42n * 10^3 s and I am unaware how to get to this. Can anybody give me some help?