You should add reference to those in your project. You should install Microsoft.AspNet.WebApi.Core package https://www.nuget.org/packages/Microsoft.AspNet.WebApi.Core/ and you will be able to use using System.Web.Http;
For System.Net.Http try to add reference to that lib by clicking right mouse button on Reference folder -> Add reference. https://stackoverflow.com/questions/9611316/system-net-http-missing-from-namespace-using-net-4-5