K9

a)
$3x^5+12x^3=3x^3\left(x^2+4\right)$
b)
$3x^5+12x^3=3x^3\left(x^2-4\right)$
c)
$x^3-2x^2+3x-6=x^2\left(x-2\right)+3\left(x-2\right)=\left(x-2\right)\left(x^2+3\right)$
d)
$x^4-1=\left(x^2+1\right)\left(x-1\right)\left(x+1\right)$