\[a_{3} = 10;\ \ a_{7} = - 6;\]
\[a_{3} = a_{1} + 2d;\ \ a_{1} = a_{3} - 2d;\]
\[a_{7} = a_{1} + 6d;\ \ a_{1} = a_{7} - 6d;\]
\[a_{3} - 2d = a_{7} - 6d\]
\[10 - 2d = - 6 - 6d\]
\[- 2d + 6d = - 6 - 10\]
\[4d = - 16\]
\[d = - 4.\]
\[Ответ:d = - 4.\]