\[- 3a^{2} - 6ab - 3b^{2} =\]
\[= - 3 \cdot \left( a^{2} - 2ab + b^{2} \right) =\]
\[= - 3 \cdot (a - {b)}^{2} = - 3 \cdot (a - b)(a - b)\]