\[\boxed{\text{590}\text{\ (590)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{n} = x_{1} + d(n - 1)\]
\[x_{16} = - 7;\ \ x_{26} = 55:\]
\[\left\{ \begin{matrix} x_{1} + 15d = - 7 \\ x_{1} + 25d = 55 \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} 10d = 62\ \ \ \ \ \ \ \ \ \\ x_{1} = - 7 - 15d \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} d = 6,2\ \ \ \ \ \ \ \\ x_{1} = - 100. \\ \end{matrix} \right.\ \]
\[Ответ:d = 6,2;\ \ x_{1} = - 100.\]