9（a-b）²-16（a+b）²
=[3（a-b）]²-[4（a+b）]²
=[3（a-b）-4（a+b）][3（a-b）+4（a+b）]
=[3a-3b-4a-4b][3a-3b+4a+4b]
=-（a+7b）（7a+b）．