\[\boxed{\text{425\ (425).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\left\{ \begin{matrix} x - 2y = 4b \\ 2x + y = 39 \\ \end{matrix} \right.\ ;\ \ \ (18;3)\]
\[\left\{ \begin{matrix} 18 - 2 \cdot 3 = 4b \\ 2 \cdot 18 + 3 = 39 \\ \end{matrix} \right.\ \]
\[18 - 6 = 4b\]
\[4b = 12\]
\[b = 3.\]
\[Ответ:при\ b = 3.\]