\[(2a - 3)^{2} - 81 =\]
\[= (2a - 3 - 9)(2a - 3 + 9) =\]
\[= (2a - 12)(2a + 6) =\]
\[= 2 \cdot (a - 6) \cdot 2 \cdot (a + 3) =\]
\[= 4 \cdot (a - 6)(a + 3)\]