\[b_{1} = 7;\ \ b_{10} = - 11:\]
\[b_{10} = b_{1} + 9d\ \ \]
\[9d = b_{10} - b_{1}\]
\[d = \frac{b_{10} - b_{1}}{9} = \frac{- 11 - 7}{9} = - 2.\]
\[Ответ:\ - 2.\]