How to map data between two Excel sheets and represent the data in another file

Question

I have two Excel files that look like this:

Asset table has firm's Id as a foreign key column, both tables have a many-to-many relationship.

Where many asset class name can have zero or many firm's id connected to it

And many firm's id can have zero or many class asset class Name.

The work here: I need to export another excel sheet with the data represented like this:

The table headers are the Asset class Names.

Starting from the second row, represent the Firm Name and Mark 'X' to the Asset Class Name that marked with the same id common.

I created another class representing called AssetFirmJoinTable, it's a join table to represent the data shared between two tables. How can I represent this data as needed? I am using ASP.NET MVC

Thanks for your help!!