This question already has an answer here:
$21= 3 \times 7$
there is only one Sylow $3$ and Sylow $7$ subgroup
so, Sylow $3$ and Sylow $7$ subgroup are normal in group $G$
so $G$ is cyclic group of order $21$.
Am I right ?
somebody told me that group of order $21$ is not cyclic.
he gave me this link.
If group of order 21 is not cyclic, then can we understand it by Sylow method ?