x_1=-\frac{\pi }{6}+\pi =\frac{5 \pi }{6}\lor  x_2=-\frac{\pi }{6}+2\pi =\frac{11 \pi }{6}\lor x_3=\frac{2\pi }{3}\lor x_4=\frac{2 \pi }{3}+ \pi =\frac{5\pi }{3}