x^4-2x^3-6x^2+6x+9
= (x^4-6x^2+9)-(2x^3+6x)
= (x^2-3)^2-2x(x^2-3)
= (x^2-3)(x^2-2x-3)
= (x+√3)(x-√3)(x-3)(x+1)