Answer:
Infinite arithmetic series.
Explanation:
In the given set of numbers 6, 9, 12, 15..........
we find the difference in each successive term is = 9 - 6 = 3
12 - 9 = 3
Since number of the terms are infinite so it's a series and there is a common difference of 3 in each successive term of the series, so set of these numbers is an Infinite Arithmetic series.