I’ve been working on this task for a while but i don’t know where to start.

Anyone that can give me any tips on how to solve it?

Light travels in such a way that it requires the minimum possible time to get from one point to another. A ray of light from C reflects of a plane mirror AB at X and then passes through D. (see figure) Show that the rays CX and XD make equal angles with the normal to AB at X. (Remark: you may wish to give a proof based on elementary geometry without using any calculus, or you can minimize the travel time on CXD.)