How to fix this issue?

Aug 11 2017 6:06 AM

Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

have changed framework version to 3.5 but not working. i search some solutions online but this is not working