Solution to that extension conflict for system.Web.Extension

If in web.congig that extension conflicts ocuurs with dll of system.web.extension you can resolve it this stuff after system.web section completion.
<runtime>

<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">

<dependentAssembly>

<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>

<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>

</dependentAssembly>

<dependentAssembly>

<assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>

<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>

</dependentAssembly>

</assemblyBinding>

</runtime>