Which are composite or prime
9 , 2 , 15 , 21

A composite number is a number that has more than 2 factors.
A prime number is a number who only have 2 factors (1, and itself)
Factors:-

9 = 1, 3, 9 = composite
2 = 1, 2 = prime
15 = 1, 3, 5, 15 = composite
21 = 1, 3, 7, 21 = composite

So, the prime #'s  in this set are 2. The composite #'s in this set are 9, 15, and 21.

Hope I helped ya!! 

A Prime Number can be divided evenly only by 1 or itself. It alsp must be a whole number greater than 1. Therefore, 9, 15, and 21 are composite and 2 is a prime.