As a rocket passes your space station, it emits a flash of light. Later, it emits a second flash, which you determine was emitted three seconds later, at which time the rocket was two light seconds away. How much time separated the two flashes according to instruments on the rocket? Hint: the easiest way is to use the invariant metric ds^2 = c^2dt^2 -dx^2-dy^2-dz^2, which is the same in all frames (replace d's by deltas if you prefer).