Man I've become desperate, can anyone help me with these two problems?

Let A be the set {1,2,3,4}. Prove that a relation R on A with 15 ordered pairs is not transitive.

I've got no clue on that one.

And this second one, which I know the proof, but I need some help wording it correctly:

If f is injective (one-to-one) and C subset D are any subsets of A, then f(D-C) = f(D) - f(C).