\[\frac{m^{2} - n^{2}}{m + 3n} + m - 3n =\]
\[= \frac{m^{2} - n^{2}}{m + 3n} + (m - 3n)^{\backslash m + 3n} =\]
\[= \frac{m² - n^{2} + m^{2} - 9n²}{m + 3n} =\]
\[= \frac{2m² - 10n²}{m + 3n}\]