21 is not a prime number, meaning that it has more then just two factors.
Factors of 21:
1, 3, 7, 21
Factors are numbers that can evenly be put into a number (without having to multiply by a decimal)
Example:
3x7=
=21
Therefore, 3 and 7 are factors of 21
Factors of 21:
1, 3, 7, 21