\[Схематический\ рисунок.\]
\[S_{\text{ABC}} = 36\ см^{2};\angle ABC = 30{^\circ}:\]
\[BC = AB;\]
\[S = \frac{1}{2}AB \bullet BC\sin{\angle ABC}\]
\[AB^{2} \bullet \sin{30{^\circ}} = 72\]
\[AB^{2} \bullet \frac{1}{2} = 72\]
\[AB^{2} = 144\ \ \ \]
\[AB = 12\ см.\]
\[Ответ:\ \ 12\ см.\]