How to get page URL in Web API controller?

Site 1: test.page.com/News

Site 2: www.page.com/News

Site 3: m.site.com/News

Hi I 'm calling a pageapi.

Web Api url:http://ift.tt/1LUsvHa

How can I get website url(http://m.site.com/) in my Getnews api controller? Which page post my api?

I try Url.Request, Request.RequestUri,HttpContext.Current.Request.Path . But I 'didn't get. Always get web api url.