The problem is this:
Let A be the set of integers and let n be a fixed positive integer. Define a relation on A be saying xRy if n divides x - y or x + y.
Just giving an example, if n = 5 we know 3R7 since 5 divides 3 + 7 = 10. Also it works with 12R2 12 - 2 = 10. Prove R is an equivalence relation.