I seem to have some problems with this one. Maybe you can check if i did it right.
Let Y ∈ N with Y ≥ 2. Show that their exist one definite X ∈ N with X + 2 = Y
Solution: I did it by a contradiction proof, by saying there is not a definite X so that X, X' are elements of N with X not the same as X'. And as we know should X+2 = X'+2 = y. However this can not be true, as also Y-2 = X = X' as we defined X is not the same as X'
Is this enough or does it make no sense.