Answer :
[tex] \bf \textit{arc's length}\\\\
s=r\theta ~~
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
r=3\\
\theta =\frac{2\pi }{7}
\end{cases}\implies s=3\cdot \cfrac{2\pi }{7}\implies s=\cfrac{6\pi }{7} [/tex]
and you can use whatever value for π, some use 3.14, and round it as needed.