I have an even easier one: any prime number can be written as n.
But I agree ... any prime number can be written as 6n+/-1.
My argument is ... any prime number can be written as 2n+1.
Actually, mine is easier in that you don't need to consider two cases -- 6n+1 and 6n-1.
Unfortunately I can't complete the proof.
The proof works for any (3p-1); I wonder why the puzzle writer chose 26.
299999 (300000-1) might be discouraging? -1 trivial? ["prime = n" works there.]