Prove that if $p$ and $8p-1$ are prime numbers then $8p+1$ is composite 
Prove that if $p$ and $8p-1$ are prime numbers then $8p+1$ is composite.

I don't know where to start... I would appreciate some hints. Thank you!

If $8p-1$ is prime, then it is not divisible by 3.
$8p$ is also not divisible by 3, since $p$ is a prime number.
However, every third number must be divisible by 3. That means that $8p+1$ is divisible by 3, which means it is composite.
