Xian Zoo - How to get there form city center?

There is no direct bus from the city center to reach the zoo.
The Tourism bus 9 can take you to the Zoo, but you need to take the bus from the Big Wild Goose Pagoda or from Television Center.
There are many buses from the city center to those two places and you can choose the suitable way.
If you are still unclear about the way to get to the zoo, you can tell me the exact place where you are staying in the city center. Then you can tell you the exact way.

How long it will take to go by taxi and how much it could cost? We will be staying near Drum/Bell Towers. Thank you.

Since you are near the Bell Tower, you can take the subway line 2 to get to its terminal and then take a taxi to go to the zoo and it is much cheaper than going from the Bell Tower. It will take you about 60-70 RMB.