residency
isn't part of the Course class, it's part of the Student class:
public class Student{
private String name;
private String studentnumber;
private String residency;
...
So you cannot access it inside the Course class unless you have an instance of the Student class to work with.
Think about it: when you calculate the the tuition cost for a course in the real world, you apply discounts according to the student's situation. Without knowing which student you are calculating for, you can't apply any discounts!
What you need to do is pass the Student instance to the
calculateTuition
method so it have use that to apply discounts.
Your other problems are because you are using the wrong method names:
courses.tuition
instead of
courses.calculateTuition
and such like.