18 has prime factors 3 & 2 whereas 21 has prime factors 3 & 7.
The factors are: 1, 3, 7, 21 The prime factors are: 3, 7
21 = 3 x 7. Both these factors are prime.
The factors of -18 are 1, 2, 3, 6, 9, 18 and their negative counterparts. The prime factors are 2 and 3.
The factors of 15 are: 1, 3, 5, and 15.The prime factors of 15 are: 3 and 5.The factors of 21 are: 1, 3, 7, and 21.The prime factors of 21 are: 3 and 7.