\[49m² + mn + 4n^{2}\]
\[49m^{2} + 28mn + 4n^{2} = (7m + 2n)^{2}\]
\[\frac{1}{16}m^{2} + mn + 4n^{2} = \left( \frac{1}{4}m + 2n \right)^{2}\]
\[49m^{2} + mn + \frac{1}{196}n^{2} =\]
\[= \left( 7m + \frac{1}{14}n \right)^{2}.\]