I am having a bit of trouble trouble understanding how to start problems such as this one. I feel like I am given information that I understand separately but I can't seem to figure out how to they relate. 

Let n ∈ Z (integers) be positive. Show that the equivalence relation 

**n|(a − b)** 

has equivalence classes 

**[r] = {kn + r|k ∈ Z}, and 0 ≤ r ≤ n − 1**

I know that if let's say n=3 that the classes would be [0]= {3k}, [1]={3k+1}, etc. the variable n is what throws me off. I appreciate any sort of starting points that would help me understand more. I assume I have to show that the class is symmetric, reflexive, and transitive, but how could I show that?