A denumerable set is a countable set (that is, it is "listable" ... you can assign a 1st element, 2nd element, 3rd element, etc...)
(a)
if A = integers and B = even integers, then they are each denumerable and
A U B = the integers, which is denumerable and
A int B = the even integers, which is denumerable
(b)
with A and B as above, then A U B is denumerable and A-B is the set of odd integers which is again denumerable.