The Calculations class does not have a constructor that takes an Entity as argument, actually it has no constructors at all except for the default generated one.
Compilers usually give you pretty detailed error messages. Posting that output along with the code makes it a lot easier for others to find the error especially if it's not as obvious as in this case.
Edit: Wow not only too slow to give the response, but also slower than the answer to that response.... it was close though ;)