Step 1 :
Factor out common factor $ \color{blue}{ 6 } $:
$$ 6a^2+6ab-12a-12b = 6 ( a^2+ab-2a-2b ) $$Step 2 :
To factor $ a^2+ab-2a-2b $ we can use factoring by grouping.
Group $ \color{blue}{ a^2 }$ with $ \color{blue}{ ab }$ and $ \color{red}{ -2a }$ with $ \color{red}{ -2b }$ then factor each group.
$$ \begin{aligned} a^2+ab-2a-2b &= ( \color{blue}{ a^2+ab } ) + ( \color{red}{ -2a-2b }) = \\ &= \color{blue}{ a( a+b )} \color{red}{ -2( a+b ) } = \\ &= (a-2)(a+b) \end{aligned} $$