I have this problem Ive been looking at for about 6 hours. It requires me to find all primes p such that 3 is a quadratic residue (mod p). All I could come up with is that every prime p ending in a 1 makes 3 a QR mod p. But this came after using excel and computing all primes. Surely there must be an easier way (assuming that my assumption is correct). The question does give a hint to use the Quadratic reciprocity law, but I have no idea what that is!