x^4-6x^2-7x-6
=x^2(x^2-4)-2x^2-7x-6
=x^2(x+2)(x-2)-(2x+3)(x+2)
=(x+2)[x^2(x-2)-(2x+3)]
=(x+2)(x^3-2x^2-2x-3)
=(x+2)[x^2(x-3)+x^2-2x-3]
=(x+2)[x^2(x-3)+(x+1)(x-3)]
=(x+2)(x-3)(x^2+x+1)