\[\boxed{\text{П.81.}}\]
\[\textbf{а)}\ 53x - 28x - 24 = 251\]
\[25x = 251 + 24\]
\[25x = 275\]
\[x = 275\ :25\]
\[x = 11.\ \]
\[Ответ:11.\ \]
\[\textbf{б)}\ (17x - 3x)\ :17 = 364\]
\[14x = 364 \cdot 17\]
\[14x = 6\ 188\]
\[x = 6\ 188\ :14\]
\[x = 442.\ \]
\[Ответ:442.\]