Dear friend:
I'm so sorry that I can't travel to Chengdu with you next weekends. I have a very important examination next weekend, and that is such a exam which will change my life. So I'd like to stay at home and review. But I promise, once I finish my exam I'll call you so that we can decide another time to travel together.
Call me soon as you get my letter.
Apologize again.
Yours
Li Hua