Solve "cos 3x = -1" algebraically given the domain [0,2pi)

Cos 3 x = - 1, given the domain [ 0, 2 π ].
3 x = π,  x 1 = π / 3 ( or : x 1 = 60° )
3 x = 3 π,  x 2 = π ( x 2 = 180° )
3 x = 5 π,  x 3 = 5 π / 3  ( x 3 = 300° ).

2,4,5,8,10 are divisible by 40