\[a_{4} = 6\sqrt{2}\ см.\]
\[1)\ R_{4} = \frac{a_{4}}{2\sin\frac{180{^\circ}}{4}} = \frac{6\sqrt{2}}{2 \bullet \sin{45{^\circ}}} =\]
\[= 3\sqrt{2}\ :\frac{\sqrt{2}}{2} = 6\ см.\]
\[2)\ a_{3} = 2R_{3}\text{\ tg}\frac{180{^\circ}}{3} =\]
\[= 2 \bullet 6 \bullet tg\ 60{^\circ} = 2 \bullet 6 \bullet \sqrt{3} =\]
\[= 12\sqrt{3}\ см.\]
\[Ответ:\ \ 12\sqrt{3}\ см.\]