When assembly added into global assembly cache and there is need for adding that assembly to web.config assemblies section.
<add assembly="System.Core, Version=184.108.40.206, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
Now there is need to add System.Web.Extensions assembly , for that you need assembly detail with its public key token to register.
Easiest way to get that detail is
1. Go to run and type assembly.
it will open following window.
2. Now select assembly , for example system.web.extensions , then go to edit menu and select option Copy display name as shown in following image.
3. Now paste data.
you get all detail to register assembly. Enjoy… !