sec (3x) = -2√3/3 = -2/√3
Notice that this is the same as
cos (3x) = -√3/2
Note that cos (x) = -√3/2 at (5/6)pi , (7/6)pi , (17/6)pi, (19/6)pi, (29/6)pi and (31/6)pi
Dividing each of these angles by 3 gives us the solutions we want =
(5/18)pi , (7/18)pi, (17/18)pi, (19/18) pi, (29/18)pi, (31/18)pi
sec (3x) = -2√3/3 = -2/√3
Notice that this is the same as
cos (3x) = -√3/2
Note that cos (x) = -√3/2 at (5/6)pi , (7/6)pi , (17/6)pi, (19/6)pi, (29/6)pi and (31/6)pi
Dividing each of these angles by 3 gives us the solutions we want =
(5/18)pi , (7/18)pi, (17/18)pi, (19/18) pi, (29/18)pi, (31/18)pi