<em>p</em> ² + 6 is even if <em>p</em> ² is even (the sum of two even numbers is even), and this happens only if <em>p</em> itself is even, i.e. if <em>p</em> = 2 since 2 is the only even prime.
All other primes <em>p</em> are odd, so <em>p</em> ² would be odd, and adding 6 would not change that (odd + even = odd).
