Could not load file or assembly 'AjaxControlToolkit, Version=3.5.40412.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies.

Hi, I'm getting the title of this thread as an error, I was trying to the AjaxControlToolkit as my assembly but I failed. I don't know how to fix this I have gone through some threads on how to fix this but still I'm getting this error. here is my whole code  of web config.
<?xml version="1.0"?> <configuration> <configSections> <sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"> <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"> <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Ex ...