\[25a^{2} - (c - 5a)(c + 5a) =\]
\[= 25a^{2} - \left( c^{2} - (5a)^{2} \right) =\]
\[= 25a^{2} - c^{2} + 25a^{2} =\]
\[= 50a^{2} - c^{2}\text{.\ }\]