\[\left( \frac{10}{21} - \frac{25}{28} \right)\ :\left( - \frac{11}{14} + \frac{24}{35} \right) =\]
\[= \left( \frac{40}{84} - \frac{75}{84} \right)\ :\left( - \frac{55}{70} + \frac{48}{70} \right) =\]
\[= \left( - \frac{35}{84} \right) \cdot \left( - \frac{70}{7} \right) =\]
\[= \left( - \frac{5}{12} \right) \cdot ( - 10) = \frac{50}{12} = \frac{25}{6} =\]
\[= 4\frac{1}{6}\]