Which bus goes to qing gong from northwestern polytechnical university in Xian?

I suggest you to take a taxi to reach Laodonglu Station where you can find the local subway line 1, so you can take the line 1 to get to Tonghualu Station. From this station, you can walk to the Qing Gong Market.
I also know a direct bus line, 512, which can take you to reach that market directly. But as I know, this line is hard to wait and the bus condition is not so nice. So I suggest you use the subway.