\[25 - m^{2} - 12mn - 36n^{2} =\]
\[= 25 - \left( m^{2} + 12mn + 36n^{2} \right) =\]
\[= 5^{2} - (m + 6n)^{2} =\]
\[= (5 - m - 6n)(5 + m + 6n)\]