\[14mn^{2} - 7n = 7n\left( \frac{14}{7}mn - 1 \right) =\]
\[= 7n(2mn - 1)\]
\[Проверка:\]
\[7n(2mn - 1) = 14mn^{2} - 7n\ \]
\[14mn^{2} - 7n = 14mn^{2} - 7n\]
\[верно.\]