\[r = \ 8\sqrt{3}\ см:\]
\[r_{n} = \frac{a_{n}}{2\text{tg}\frac{180{^\circ}}{n}}\ \]
\[a_{n} = 2r_{n}\text{tg}\frac{180{^\circ}}{n}\]
\[a_{6} = 2 \bullet 8\sqrt{3} \bullet tg\ 30{^\circ} =\]
\[= 16\sqrt{3} \bullet \frac{\sqrt{3}}{3} = 16\ см.\]
\[Ответ:\ \ 16\ см.\]