Is 5,9,15 an arithmetic sequence? Explain

5, 9, and 15 is not an arithmetic sequence because

9 - 5 = 4

15 - 9 = 6

4 and 6 do not equal each other

If the numbers were 5, 9, and 13, then it would be an arithmetic sequence with a common difference of 4. In this case, there is not common difference because the sum of the numbers keep changing.