For set difference .
Recall that the complement of a closed set is an open set and the intersection of two open sets is an open set.
Recall that the complement of an open set is a closed set and the intersection of two closed sets is a closed set.
This is my own take on this.
These are questions about basic topology.
I do not think that topics in topology have any place in a course on discrete mathematics.
Your instructor gave you something beyond your grasp.
The sad truth is: it may enhance her/his on importance without doing anything for you.