\[\boxed{\mathbf{730\ (730).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[x_{8} = 58,;\ x_{15} = 16\]
\[\left\{ \begin{matrix} x_{1} + 7d = 58\ \ \\ x_{1} + 14d = 16 \\ \end{matrix} \right.\ ( - )\text{\ \ \ \ }\]
\[\ \left\{ \begin{matrix} - 7d = 42\ \ \ \ \ \\ x_{1} = 58 - 7d \\ \end{matrix} \right.\ ,\ \ d = - 6\]
\[Ответ:\ d = - 6.\]