$$\sin ( - \frac { x } { 3 } ) = \frac { \sqrt { 2 } } { 2 }$$
$x=6\pi n_{1}+\frac{15\pi }{4}\text{, }n_{1}\in \mathrm{Z}$
$x=6\pi n_{1}+\frac{21\pi }{4}\text{, }n_{1}\in \mathrm{Z}$
