Answer
Composite number
Work Step by Step
A $\textit{prime}$ number is a natural number greater than $1$ which has only two factors: $1$ and the number itself. In case it has more than two factors, it is a $\textit{composite}$ number.
In order to determine if the given number is prime, we determine its factors.
$21=1\cdot 21=3\cdot 7$.
Because $21$ is divisible by $1$, $3$, $7$ and $21$, therefore by more that $1$ and itself, it follows that it is a composite number.
