D(x)=x^4+7x^3+6x^2 D(x)=x^3-3x^2-3x+9
x^4+7x^3+6x^2=x^2(x^2+7x+6)
\Delta=49-24=25
x_1=\frac{-7-5}{2}=-6
x_2=\frac{-7+5}{2}=-1
D9x)=x^2(x+1)(x+6)
x^3-3x^2-3x+9=x^2(x-3)-3(x-3)=
=(x-3)(x^2-3)=(x-3)(x-\sqrt3)(x+\sqrt3)