=9876*6788+9876-9877*6788
=（9876-9877）*6788+9876
=9876-6788
=3088
好想你们还没有学到负数,这样
=（9877-1）*6789-9877*6788
=9877*6789-6789-9877*6788
=9877*（6789-6788）-6789
=9877-6789
=3088