\[\frac{2m - 4n}{21c} + \frac{5m + 18n}{21c} =\]
\[= \frac{2m - 4n + 5m + 18n}{21c} =\]
\[= \frac{7m + 14n}{21c} = \frac{7 \cdot (m + 2n)}{21c} =\]
\[= \frac{m + 2n}{3c}\]