You could write it as (2^k)^k - 1. Can you think of a way to factorize it now?
By the way, primes of the form 2^n - 1 are called Mersenne primes. They're relatively well-known, and have many unsolved problems associated to them. For example, are there infinitely many Mersenne primes?
And I apologize for not showing my work, but most (nearly all) of it has been erased at this point, and it all was deadends. Thanks for your help guys. I'll let you know if/when I get an answer. Or you could put me at peace before that.