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, we have $p \neq -1 \mod 3$.
This leaves us with $p=3$ (do this case by hand) or $p = 1 \mod 3$, but then $8p+1 = 0 \pmod 3$.
