x³+9x²+23x+15
=x³+x²+8x²+8x+15x+15
=x²(x+1)+8x(x+1)+15(x+1)
=(x+1)(x²+8x+15)
=(x+1)(x²+8x+16-1)
=(x+1)[(x+4)²-1²]
=(x+1)(x+4+1)(x+4-1)
=(x+1)(x+5)(x+3)