This is from chapter on linear congruences:

Show that if p is an odd prime and a is a positive integer not divisible by p, then the congruence $\displaystyle x^2 \equiv a \pmod {p}$ has either no solution or exactly two incongruent solutions.

I can see If $\displaystyle p \mid x$, then there are no solutions. I'm really not sure how to approach the rest of this...