$a)$

$w\left(x\right)=\left(20x^3-28x^2+8x\right)\left(x^4+6x^3+2x^2+12x\right)=$

$=\left(4x\left(5x^2-7x+2\right)\right)\cdot \left(x^4+2x^2+6x^3+12x\right)=$

$=\left(4x\left(5x^2-7x+2\right)\right)\cdot \left(x^2\left(x^2+2\right)+6x\left(x^2+2\right)\right)=$

$=\left(4x\left(5x^2-7x+2\right)\right)\cdot \left(\stackrel{\Delta =0-8<0}{\left(x^2+2\right)}\left(x^2+6x\right)\right)=$

$=4x\stackrel{\left(\ast \right)}{\underline{\underline{\left(5x^2-7x+2\right)}}}\left(x^2+2\right)\left(x+6\right)x=\dots$