I think you mean Xian.
You'd better have an English guide since lots of locals, drivers...understand little English. What is your flight departing time? If you have a late afternoon flight to Hongkong, your trip sounds possible but tired. If you arrive in Xian early in the morning around 8am then you could take the tour special line 5 or 306 at the east square of the railway station to the Terracotta. Later, you could return to Xian around 2pm or 3pm. Also at the railway station, you could find the shuttle to the airport.