The distance between 2 spheres in pendulum

[Dr.Phy]

Hi guys,first nice to meet you I am new here.Im working on a physics homework and i.ve already done it but i don't know why the x=2lsinx ( x=distance between 2 spheres in pendulum).
(P.S sorry for my bad english,im from Germany). Is that from a trig identity? I think I've understand it but if someone can tell mi with a pic it will be very helpful.

 

[WillemBouwer]

just ad a pic of your problem and also your attemot at a solution and we can figure some solution out