$\sqrt{3}sin(2x)-cos(2x)=-2cox(x)$$\frac{\sqrt{3}}{2}sin(2x)-\frac{1}{2}cos(2x)=-cos(x)$
$sin(2x)sin(\pi /3)-cos(2x)cos(\pi /3)=-cos(x)$
$cos(2x)cos(\pi/3)-sin(2x)sin(\pi/3)=cos(x)$
$cos(2x+\pi/3)=cos(x)$
$2x+\pi/3=x+k2\pi$ hay $2x+\pi/3=-x+k2\pi$ với $k\in Z$
$x=-\pi/3+k2\pi$ hay $x=-\pi/9+k2\pi/3$ với $k\in Z$