\[a_{5} = a_{1} + 4d\]
\[a_{7} = a_{1} + 6d\]
\(a_{8} = a_{1} + 7d\)
\[a_{10} = a_{1} + 9d\]
\[11d - 15 = 15d\]
\[4d = - 15\]
\[d = - \frac{15}{4} = - 3,75.\]
\[Ответ:d = - 3,75.\]