Verified answer

Wow, vague question. I'm assuming the parts they left out are:

* "returned to the mirror" means: "returned to the REVOLVING mirror"

* "time interval elapsed" means: time it took for the round trip, from the revolving mirror to the stationary mirror and back again.

In that case, the distance for the round trip is: 24 km.

Now use this formula that you (should have) learned by 6th grade:

speed = distance / time

Rearrange:

time = distance / speed

(Hint: time = 24km / (speed of light))