Hello, I am doing my homework to study for my exam but I am stuck on this problem...plz help me

Thank you in advance.

Let R denote the real numbers, and let P be the binary relation on R X (R - {0} ) such that (x,y) P (z,w) if and only if xw = yz. Prove that P is an equivalence relation, and show that every equivalence class contains a unique element (or representative) of the form (r,1).