\[(7a + k)(k - 7a) - 3 \cdot (a + 5k)^{2} + 30ak =\]
\[= k^{2} - 49a^{2} - 3 \cdot \left( a^{2} + 10ak + 25k^{2} \right) + 30ak =\]
\[= k^{2} - 49a^{2} - 3a^{2} - 30ak - 75k^{2} + 30ak =\]
\(= - 74k^{2} - 52a^{2}\)