Automatically populating a Textbox on the basis of the value of another one

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ASP.NET

SQL-Server-2008R2

MVC4

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as title says how can i populate a Textbox on the basis of the value of another one,
first textbox is txtTasks now i want to populate project(s) which is related to that tasks in second textbox..
here is the code of linq for both,but remember i am using database not pre-list for it in asp.net mvc4 using jquery ajax..here home controller function is:

public JsonResult LoadProjectByTaskId(int taskId)
       {
           int id = 0;

           var projects = pm.GetProjectByTaskId(id);
           var result = from t in context.Tasks
                        join p in context.Projects on t.ProjectId equals p.ProjectId
                        where t.TaskId == taskId
                        select p;
          return Json(result, JsonRequestBehavior.AllowGet);

plz guide me..