First of all I must say that I don’t think this is a wordpress related question. That said I hope this following suggestion helps you go in the right direction…
Basically the include("xxx-xxx.php"); already tells you where to put the xxx-xxx.php – in this case it should go into the same directory where the ‘parent’ is located.
If instead it looked like this: include("incl/xxx-xxx.php"); then the xxx-xxx.phpshould instead go into the subdirectory “incl”