Question

suppose u= {3,6,9,12,15,18,21} is the universal set and M={3,9,15,21} what is M?

Answer 1

Are you asking for a name? M is a subset of u. Is there something further?

Answer 2

12 and 18 maybe. I'm not sure but I think this is it because those numbers are missing from M's sequence
. I might be wrong. Just guessing