This is the 917th most frequent Polish word.
którymi
"Którymi" means "which" or "whom" in instrumental plural form.
In this sentence, 'którymi' is used as an instrumental case relative pronoun referring to 'książki' (books).
To są książki, z którymi się zapoznałem podczas studiów.
These are the books with which I became acquainted during my studies.
Here, 'którymi' refers to 'trudnościami' (difficulties) and denotes the instrumental case for dealing with these difficulties.
Zastanawiam się nad trudnościami, z którymi musieliśmy się zmierzyć.
I am reflecting on the difficulties with which we had to contend.
In this example, 'którymi' is instrumental and relates to 'zadania' (tasks), indicating what will be worked upon.