Step 1 :
After factoring out $ -2x $ we have:
$$ -2x^{4}-6x^{3}+8x^{2}+24x = -2x ( x^{3}+3x^{2}-4x-12 ) $$Step 2 :
To factor $ x^{3}+3x^{2}-4x-12 $ we can use factoring by grouping:
Group $ \color{blue}{ x^{3} }$ with $ \color{blue}{ 3x^{2} }$ and $ \color{red}{ -4x }$ with $ \color{red}{ -12 }$ then factor each group.
$$ \begin{aligned} x^{3}+3x^{2}-4x-12 = ( \color{blue}{ x^{3}+3x^{2} } ) + ( \color{red}{ -4x-12 }) &= \\ &= \color{blue}{ x^{2}( x+3 )} + \color{red}{ -4( x+3 ) } = \\ &= (x^{2}-4)(x+3) \end{aligned} $$Step 3 :
Rewrite $ x^{2}-4 $ as:
$$ x^{2}-4 = (x)^2 - (2)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = x $ and $ II = 2 $ , we have:
$$ x^{2}-4 = (x)^2 - (2)^2 = ( x-2 ) ( x+2 ) $$