a﹙x－y﹚－3ab﹙y－x﹚ ＝a[a﹙x－y﹚]＋﹙3b﹚[a﹙x－y﹚] ＝﹙a＋3b﹚ [a﹙x－y﹚] ＝ a ﹙a＋3b﹚ ﹙x－y﹚