How to fix the type or namespace name 'customerbal' could not be found

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ASP.NET4

How to fix error "the type or namespace name 'CustomerBAL' could not be found (are you missing a using directive or an assembly reference)

Name project MvcApplication1) on ASP.Net Visual Studio 2012
Could anyone help me ? Thank's in advance ...

Here is the coding on TestController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Dynamic;

namespace MvcApplication1.Controllers
{
public class TestController : Controller
{
public string MyFirstMethod()
{
return "Welcome ToString Queenspoted" ;
}

public ActionResult MyFirstViewMethod()
{
CustomerBAL b = new CustomerBAL();
Customer c = b.GetCustomer();

ViewBag.CustomerData = c;
return View("MyFirstView");
}
}
}

What I have tried:

I had changed targed framework from .Net Framework 4.5 to .Net Framework 4.0.But it does not work...