[tex]\bf \textit{arc's length}\\\\
s=r\theta \quad
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
s=6\pi \\
\theta =\frac{\pi }{3}
\end{cases}\implies 6\pi =r\left( \frac{\pi }{3} \right)\implies \cfrac{6\pi }{\frac{\pi }{3}}=r
\\\\\\
\cfrac{6\pi }{1}\cdot \cfrac{3}{\pi }=r\implies 18=r[/tex]
