\[a_{6} = - 17,5;a_{11} = - 37;\]
\[a_{6} = a_{1} + 5d;a_{1} = a_{6} - 5d;\]
\[a_{11} = a_{1} + 10d;a_{1} = a_{11} - 10d;\]
\[a_{6} - 5d = a_{11} - 10d\]
\[- 17,5 - 5d = - 37 - 10d\]
\[5d = - 19,5\]
\[d = - 3,9.\]
\[Ответ:d = - 3,9.\]