\[36a^{4} - 25a^{2}b^{2} =\]
\[= \left( 6a^{2} - 5ab \right)\left( 6a^{2} + 5ab \right) =\]
\[= a \cdot (6a - 5b) \cdot a(6a + 5b) =\]
\[= a²(6a - 5b)(6a + 5b)\]