(a) Derive equation (3.4).
(b) According to clocks on the ground (system S), streetlights A and B (situated 4 km apart) were both turned on at precisely 8:00 p.m. Which one went on first according to an observer on a train (system S′), which moves from A toward B at 53 the speed of light? How much later (in seconds) did the other light go on? [*Note:* As always in relativity, we are talking here about what S′ *observed*, after correcting for the time it took the light to reach her, not what she actually *saw* (which would depend on where she was located on the train).]