\[\boxed{\mathbf{756}\mathbf{\ (584)}.\ Еуроки\ - \ ДЗ\ без\ мороки}\]
\[a_{кв} = 12\ см;\ \ \ a_{пр} = 18\ см;\ \]
\[\ S_{кв} = S_{пр}\]
\[S_{кв} = a^{2} = 12^{2} = 144\ см^{2}.\]
\[S_{пр} = ab = 18 \cdot b = 144\ см^{2}.\ \]
\[b = 144\ :18 = 8\ см.\]
\[P_{пр} = 2 \cdot (a + b) =\]
\[= 2 \cdot (18 + 8) = 2 \cdot 26 = 52\ см.\]
\[Ответ:52\ см.\]