Entity-Framework is in costant development, and you may be using it in many projects.
So, you may be using different versions.
There are many ways to check which version is currently in use
Package Manager Console
And use the command: Get-Package
Expand the Reference section and select Properties
There you can find the Entity-Framework you are using.
Open the packages.config file in the project root.
Search for the Entity-Framework row in this file.