Equivalence class over a set of natural numbers with conditions

Question

My discrete math class has this problem: we need to find the equivalence classes for the equivalence relation over the set of $\mathcal{N}$ onto $\mathcal{N}$ defined as follows: $$xRy \iff (x-3.3)(y-3.6)>0$$ I thought that there's only 1 class: $\{\mathcal{N}-{0,1,2,3}\}$ that is all natural numbers greater than 3. However the solution says that there's another equivalance class: $\{0,1,2,3\}$. But how can this be? $$1R2 \to (1-3.3)(2-3,6) <0$$ so clearly such input renders the output to be below zero hence it can't be part of the relation.

Answer 1

$$(1-3.3)(2-3.6)\color{red}{>}0$$ or more generally $$x\cdot y>0\iff (x>0\ \land y>0)\ \lor \ (x<0\ \land y<0)$$