Find set A and B (neither are empty)

Question

we want A is a subset of B to be true and
A is an element of B is true.
A ={3} and B = {3, 4}
So A is a subset of B
But now I am worried about the notation for A to be an element of B.
I think of have seen some sets with sets inside of sets.
Do I have to write B as {{3},{4}}? Is that right?

Answer 1

Now, I think the answer is A ={3} and B could = {3, {3}, 5)

Answer 2

unusual, but works for me