# How do you factor 3x²-12xy-63y²?

Given $3 {x}^{2} - 12 x y - 63 {y}^{2}$
Extract the obvious common numeric factor $3$
$\textcolor{w h i t e}{\text{XXXX}}$$= 3 \left({x}^{2} - 4 x y - 21\right)$
$21$ can be factored as $3 \times 7$ and $3 - 7 = - 4$
$\textcolor{w h i t e}{\text{XXXX}}$$= 3 \left(x + 3\right) \left(x - 7\right)$