x^3+2x^2-5x-6=x^3+x^2＋x²-5x-6
=x²﹙x＋1﹚＋﹙x＋1﹚﹙x－6﹚
=﹙x＋1﹚﹙x²＋x－6﹚
=﹙x＋1﹚﹙x＋3﹚﹙x－2﹚