正确结果是把错误的结果加上应该减去的那个多项式的2倍
-2bc-3ac+2ab-2(ab-2bc+3ac)
=-2bc-3ac+2ab-2ab+4bc-6ac
=2bc-9ac
如果错误结果是2bc-3ac+2ab
那么正确结果是：
2bc-3ac+2ab-2(ab-2bc+3ac)
=2bc-3ac+2ab-2ab+4bc-6ac
=6bc-9ac