$$\eqalign{
& \cos 3x = \cos \left( { - {{5\pi } \over 7}} \right) \cr
& \Leftrightarrow \left[ \matrix{
3x = - {{5\pi } \over 7} + k2\pi \hfill \cr
3x = {{5\pi } \over 7} + k2\pi \hfill \cr} \right. \cr
& \Leftrightarrow \left[ \matrix{
x = - {{5\pi } \over {21}} + {{k2\pi } \over 3} \hfill \cr
x = {{5\pi } \over {21}} + {{k2\pi } \over 3} \hfill \cr} \right.\,\,\left( {k \in Z} \right) \cr} $$
