x^4-2x^3-6x^2+6x+9=0
x^4-2x^3-3x^2-3x^2+6x+9=0
x^2(x^2-2x-3)-3(x^2-2x-3)=0
(x^2-2x-3)(x^2-3)=0

(x+1)(x-3)(x^2-3)=0