Answer:
21 = 3 x 7
Explanation:
If Sam sent three friends an email on Monday and each friend sent an email to 3 friends on Tuesday and total of 3+3*3= 12 emails were sent by Tuesday. on Wednesday each of the friends that received those emails sent it to 3 friends so the total amount of emails sent by Wednesday is:

21 emails were sent by Wednesday. The prime factorization of 21 is 3 x 7