1. 表示歉意； 2.说明原因； 3.另约时间。

注意：

1. 词数100左右； 2.可以适当增加细节，以使行文连贯。

【答案】Dear Tom,

I’m sorry to say that I cannot go to the library with you on Friday afternoon. I have just found that I have to attend an important class meeting that afternoon. I hope the change will not cause you too much trouble.

Shall we go on Saturday morning? We can set out early so that we’ll have more time to read. If it’s convenient to you, let’s meet at 8:30 outside the school gate. If not, let me know what time suits you best. I should be available any time after school next week.

Yours,

Li Hua