Packing, calculation of density

Physics, Solid State Physics
Year 4
Packing, calculation of density, etc,
--------------------------------------------------------------------------------
I do not know how to show this problem exclusively to you but I just put your name on tope instead.  Please write very clearly so that I can read the material well.  Please be very detailed especially in math because my mathematical skills are very weak.  All the problems I need to know how to solve are on the attachment.  Please only solve problem 3 on the attachment.  .      

(See attached file for full problem description)