Dear Mr Gao,
I was very glad to get your letter.Thank you for your invitation.I'd like to go to your beautiful school,but I just couldn't.My wife and I are going to a friend's birthday that will happen to be on December 7.At the end of next month,we are to fly back to New York.I really feel sorry to lose this good chance to enjoy myself with you.
Best wishes.
Yours sincerely,
Chapman
绝对准确,请放心,这是范文哦!绝对是答案!