how to go Yangzhou from Xian ? how long ? cost ?

You can travel by train and by air.

By train, it takes about 18 hours and only two trains are scheduled every day. Well, the fare is CNY173.5-181.5 for a hard seat, CNY306-341.5 for a hard sleeper and CNY464.6-522.5 for a soft sleepr.

By air, it takes about 1 hour and 50min. But the fare varies from time to time. It is suggested to refer to the website to know the exact fee according to your travel date:
https://www.travelchinaguide.com/china-flights/