3a²-2a+4a²-7a =﹙3＋4﹚a²＋﹙－2－7﹚a=7a²－9a
1/2st+4+3st-4=3.5st
2（2ab+3a）-3（2a-ab﹚=4ab＋6a－6a＋3ab=7ab
a²-【-4ab+（ab-a²）】-2ab
=a²－﹙－4ab＋ab－a²﹚－2ab
=a²＋3ab＋a²－2ab
=2a²＋ab