Write out explicitly:
1. The definition of an upper bound.
2. The definition of a supremum.
It will help you think this through more clearly.
An upper bound: If a set A⊆R a real number s is an upper bound for A if for all x A, x s.
a supremum: a real number s is least upper bound for A if s is an upper bound for A having the property that, if b is also an upper bound for A then s b.
So am I showing that there is a y such that y>s- or is that still just the definition. I'm sorry I think it is the wording of the question that is throwing me off.