To factor $ 3x^{3}-x^{2}-18x+6 $ we can use factoring by grouping:
Group $ \color{blue}{ 3x^{3} }$ with $ \color{blue}{ -x^{2} }$ and $ \color{red}{ -18x }$ with $ \color{red}{ 6 }$ then factor each group.
$$ \begin{aligned} 3x^{3}-x^{2}-18x+6 = ( \color{blue}{ 3x^{3}-x^{2} } ) + ( \color{red}{ -18x+6 }) &= \\ &= \color{blue}{ x^{2}( 3x-1 )} + \color{red}{ -6( 3x-1 ) } = \\ &= (x^{2}-6)(3x-1) \end{aligned} $$