Answer :
[tex]\bf \textit{arc's length}\\\\ s=r\theta ~~ \begin{cases} r=&radius\\ \theta =&angle~in\\ &radians\\ \cline{1-2} r=&10\\ \theta =&\frac{2\pi }{3} \end{cases}\implies s=10\left( \cfrac{2\pi }{3} \right)\implies s=\cfrac{20\pi }{3}\implies s\approx 20.94[/tex]