The union of two sets is defined as all elements that are members of both sets.
The first set evaluuates to (...,...,...-7,-6,-5,-4) and the second eveluates to (6,7,8,9,...,...).
There are no elements that belong to both sets so the solution is the empty set. {∅}
Another way is to think about it in words: "What number(s) are less than -3 and also greater than 5?" There are no numbers that fit both of these.