x^4-6x^3+9x^2-16
=x^2(x^2-6x+9)-16
=x^2(x-3)^2-16
=[x(x-3)+4][x(x-3)-4]
=(x^2-3x+4)(x^2-3x-4)
=(x^2-3x+4)(x-4)(x+1)