Don't worry, it's free.
We have already talked about prepositional supplements in chapter four of this course, more precisely about interrogatives like "womit", "wofür" or "worin". However, this has only been one part of this topic. You may have come across words like "damit", "darin" or "darin", which are pronominal constructions that we can use, i.e. when answering. Sounds complicated? Don't worry, it's not so bad!