设M＝｛a，b，c｝，N＝｛－2,0,2｝,则从M到N的映射种数为             

书面表达。

      根据下列材料，写一张留言条：假设你叫李华，你的同班同学汤姆曾叫你今天晚上到他家去帮他学

习汉语，但你母亲突然生病了，已经送往医院，你要去医院照顾她。你到汤姆家去，但他不在家，请你

据此写一张留言条，告诉汤姆改天再去帮助他。格式已给出：

Dear Tom:                                                                                                       4:00 pm, Apr.25

     This afternoon ,I went to you home but you were not in _____________________________

______________________________________________________________________________

______________________________________________________________________________

                                                                                                                 Yours

                                                                                                                 LiHua