Where is System.Web.Extensions, Version=3.6.0.0 ?

Hi all,
I'm new to ASP.NET. I have Visual Studio 2008 with .Net Framework 3.5 on my machine.
I have a project using System.Web.Extensions 3.6.0.0. I cannot run this project because I only have the version 3.5.0.0.
Can you please tell me where I can get System.Web.Extensions 3.6.0.0?
Thank you!

Here is a part of my web.config file:
<system.webServer> <validation validateIntegratedModeConfiguration="false"/> <modules runAllManagedModulesForAllRequests="true"> <remove name="ScriptModule"/> <remove name="DynamicDataModule"/> <remove name="UrlRoutingModule"/> <add name="ScriptModule" preCondition="managedHandler" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.6.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/> <add name="DynamicDataModule" preCondition="managedHandler" type="System.Web.DynamicD ...