(3,5,7,9,11)U(3,6,9,12,15)would be correctly expressed as

5,6

To correctly express the union of the sets (3,5,7,9,11) and (3,6,9,12,15), you would write it as:

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

To correctly express the union of the sets (3, 5, 7, 9, 11) and (3, 6, 9, 12, 15), you can simply combine the elements from both sets without including any duplicates.

Here's how you can do it step by step:

1. Write down the elements of the first set: (3, 5, 7, 9, 11).
2. Now, write down the elements of the second set, ignoring any duplicates: (6, 12, 15).
3. Combine the two sets: (3, 5, 7, 9, 11, 6, 12, 15).

So, the correct expression for the union of the two sets is (3, 5, 7, 9, 11, 6, 12, 15).