Find all odd primes p for which 7 is a quadratic residue mod p.
I started out with the legendre symbol (7 / p)L = (-1)^(((7-1)/2)*((p-1)/2))*(p / 7)L = (-1)^(3(p-1)/2)*(p / 7)L by the quadratic reciprocity law, and I know I want it to be equal to 1. Now I'm not sure where to go from here.