-6(x-y)³+18(y-x)²-24(y-x)
＝﹣6﹙x－y﹚³＋18﹙x－y﹚²＋24﹙x－y﹚
＝﹣6﹙x－y﹚[﹙x－y﹚²－3﹙x－y﹚－4]
＝﹣6﹙x－y﹚﹙x－y－4﹚﹙x－y＋1﹚.