Hi,

"If p is a prime congruent to 3 (mod 4) and $\displaystyle x^2 + y^2$ is congruent to 0 (mod p), show p divides x and p divides y."

I tried to solve by contradiction, saying that if p doesn't divide x then it doesn't divide y either, but from there I don't know what to do. This is supposed to be a pretty straightforward problem, so any help would be great. Thanks.