x^4-2x^3-11x^2+12x+36
=(x^4-x^4-6x^2)-(x^3-x^2-6x)+6(x^2-x-6)
=x^2(x^2-x-6)-x(x^2-x-6)+6(x^2-x-6)
=(x^2-x-6)^2
＝(x+2)(x+2)(x-3)(x-3)