Answer:
well when you hear "union" you think 'joined' or 'together' since thats what it means, In a mathmatical set of numbers, union means that both of the sets are necessary for the equation to be true. I'm not sure what the sets are in this problem but in order to get "union" from anything, that means that when you checked for your domains, the values that you got had to be part of both the given sets