Can I add the values of two fields and display them in a third one?

[michibk]

I have added two new fields to the opportunities tab. I simply want to add these two values to be shown in a third field.
I saw that the new fields created are stored in the opprtunities_cstm table. I am lost since i can't find the php pages responsable for this queries. I don' know if I have to do this change to the db and how build the right query?