Question

Show that any integer of the form 6k + 5 is also of the form 3j + 2, but not
conversely

Answer 1

Let b = 2n + 1, n ∈ N. 3(2n+1) +2 = 6n + 5. However for the same type of conjecture to work on the converse you would need some m ∈ Q thus m ∉ N.

You could use Z as your subset if you want, since it did ask for integers but whatever--same thing.