ASP.NET MVC, how to correctly compose a database query?

Question

C++ / C#

Table 1 - Forum :

1. idForum (key)
   
2. Name
   

Table 2 - Topic :

1. idTopic(key)
   
2. NameTopic
   
3. idForum (related tables).
   

How to correctly compose a query to display the values ​​of the Topic table by idForum?
At the moment the request is:

return _db.Topic.Where(it => it.IdForum == id).Take(5);

Gives an error: "Unable to implicitly convert type. Explicit conversion exists (probably missing a cast)"

Answer 1

And the method from which return is made exactly has a return type IQueryable<Topic>? And in the future, exactly is the processing of IQueryable, and not IEnumerable?