What are the solutions to the equation on [0, 2π)?

First    sec = 1/ cos      to have a negative sec   cos must be negative (Q II and III)

- 2 sqrt 3/3  is the same as  -2/sqrt 3    and this is   1/ cos

     so cos (3x) = -sqrt 3/2

               3x = 5pi/6     and    7pi/6     

                  x = 5 pi/18            7 pi/18