$a)W\left(x\right)=3x^3-15x^2=$  

$=3x^2\cdot x-3x^2\cdot 5=$  

$=3x^2\left(x-5\right)$  

$b)W\left(x\right)=12x^4+6x^3=$  

$=6x^3\cdot 2x+6x^3\cdot 1=$  

$=6x^3\left(2x+1\right)$  

$c)W\left(x\right)=\left(x+2\right)\cdot 7x-4x-8=$  

$=7x^2+14x-4x-8=$  

$=7x^2+10x-8=\dots$  

$\Delta ={10}^2-4\cdot 7\cdot \left(-8\right)=$  

$=100+224=324$  

$\sqrt{\Delta }=\sqrt{324}=18$  

$x_1=\frac{-10-18}{2\cdot 7}=$   $-\frac{28}{14}=-2$  

$x_2=\frac{-10+18}{2\cdot 7}=\frac{8}{14}=\frac{4}{7}$  

$\dots =7\cdot \left(x-\left(-2\right)\right)\cdot \left(x-\frac{4}{7}\right)=$   

$=7\cdot \left(x+2\right)\cdot \left(x-\frac{4}{7}\right)$  

$d)W\left(x\right)=\left(3x-1\right)\cdot x+{\left(3x-1\right)}^2=$  

$=\left(3x-1\right)\cdot x+\left(3x-1\right)\cdot \left(3x-1\right)=$  

$=\left(3x-1\right)\cdot \left[x+\left(3x-1\right)\right]=$  

$=\left(3x-1\right)\cdot \left[x+3x-1\right]=$  

$=\left(3x-1\right)\cdot \left(4x-1\right)$