In two days we're going to Zhangjiajie and afterwards we have to go to HK to catch our flight out.

You may fly from ZJJ to Changsha (about 1 hour). Then, take a flight to Hong Kong (about 2 hours).

Or, you may take a bus to Changsha West from ZJJ Central Bus Station (about 4 hours). The buses are available from 7:00 to 17:00, and the fare is CNY 110. Then, take shuttle bus West Bus Station Line to the airport for CNY 30.5 (about 70 minutes). Upon arrival, take a flight to Hong Kong (about 2 hours).

You can choose the way you like. Have a good trip! :)