Question

(3,5,7,9, 11) U (3,6,9,12,15)

Answer 1

Just combine their elements.
And no double-entries XD

For instance
{2, 3, 4} U {1, 3, 5}
= {1,2,3,4,5}
(I didn't list down 3 twice)

Answer 2

combine all the elements and one element should not be repeat itself

Answer 3

Thank you.