\[\boxed{\mathbf{253}\mathbf{.}}\]
\[P_{\text{ABC}} = 27\ см;\]
\[P_{\text{ADC}} = 22\ см;\]
\[P_{\text{ABCD}} = 31\ см;\]
\[AC - ?\]
\[P_{\text{ABCD}} = P_{\text{ABC}} + P_{\text{ADC}} - 2AC\]
\[31 = 27 + 22 - 2AC\]
\[49 - 2AC = 31\]
\[2AC = 49 - 31\]
\[2AC = 18\]
\[AC = 18\ :2 = 9\ см.\]
\[Ответ:9\ см.\]