x^3+5x^2+3x-9
=(x^3-1)+(5x^2+3x-8)
=(x-1)(x^2+x+1)+(5x+8)(x-1)
=(x-1)(x^2+x+1+5x+8)
=(x-1)(x^2+6x+9)
=(x-1)(x+3)^2