A farmer in china discovers a mammal hide that contains 25% of its original amount of c-14

You have left a LOT out of your question. However I'll attempt to post an answer.

The half-life of carbon 14 is 5,700 years.

If there is 25% of the original carbon 14 remaining that means the time it has undergone has been 2 half lives.

2 half-lives of carbon14 = 2 * 5,700 = 11,400 years.