A lab and its two equidistant mirrors (M1 and M2) moves to the right with speed v. There is also an outside observer "at rest".
Inside the lab nothing special happens, a flash of light goes to the mirrors and comes back at the lab simultaneously from both.
Light in vacuum has the same speed c for all observers, lab and outside.
1. Compute the time for light to get from the lab to the upper mirror (or vice versa) as seen from outside, compared to the time measured in the lab.
The lab with its mirrors can be considered a clock.
Needs no more algebra than what you presumably learned when ~ fifteen; and the knowledge that speed = distance/time. Special relativity is elementary.