Determine the prime factors of 21: 21

Step 1 ：Determine the prime factors of 21.

Step 2 ：To find the prime factors of 21, first find all the prime numbers that can divide 21 without leaving a remainder. Then, divide 21 by those prime numbers until the quotient is 1.

Step 3 ：number = 21

Step 4 ：factors = [3, 7]

Step 5 ：Final Answer: The prime factors of 21 are \(\boxed{3 * 7}\).