The set of {-3,0,3} closed under subtraction

Answer 1

For a set to be closed under subtraction the following must be true:

If a and b are any two elements of the set, then a - b is also in the set.

The set {3, 0, -3} is not closed under subtraction since 3 and -3 are in the set, but 3 - (-3) = 6 is not in the set.