【正确答案】(Student's sample)
Prof Wang,
I'm sorry I can't go to your office as we said at four. My meeting will go a lot longer than I expected. I like to meet you in your office at 3 tomorrow afternoon. If you can't, please leave a note in my mailbox # 123 tomorrow morning, or call me at 659812**. Thank you.

(Improved sample)

Dear Prof Wang,
I'm sorry I couldn't make our 4 o'clock appointment in your office. My meeting may last a lot longer than I expected. Could I meet you in your office at 3 tomorrow afternoon, if it is convenient for you? If not, please leave a note in my mailbox # 123 tomorrow morning, or call me at 659812* * Thank you.