For what values of $\displaystyle {x}$ does $\displaystyle {f{{\left({x}\right)}}}={2}{\cos{{x}}}-\sqrt{{{3}}}\cdot{x}$ have a horizontal tangent line?

Give all answers in the interval $\displaystyle {0}\le{x}\lt{2}\pi$. Give your answers as exact values, separated by commas.

$\displaystyle {x}$ = Preview Question 6