A prime factorization consists in write down a multiplication in prime numbers. In this case we have that Sam sent an email to 3 friends on Monday, then all his friends an email to 3 friends more on Tuesday and the same happen on Wednesday. So this could be write as:
Monday: 3 emails
Tuesday: 9 emails
Wednesday: 27 emails
The question is to write the number of emails sent on Wednesday, so:
Is not enough because it has to be write in prime numbers:
Now the factorization is write in prime numbers. The answer is 3x3x3.