<s>[INST] The number of incongruent solutions of $x^4-14x^2+36\equiv 0\pmod{2019}$ I was asked to find the number of the incongruent solutions of $x^4-14x^2+36\equiv 0\pmod{2019}$, and the fact $2019=673\times 3$ is given.
My attempt: Given congruence is equivalent to
$$
\begin{cases}
x^4-14x^2+36\equiv 0\pmod{673}\\
x^4-14x^2+36\equiv 0\pmod{3}
\end{cases}
$$
The second one is easily reduced to $1-14\equiv 0\pmod{3}$ if $3\nmid x$, so $x\equiv 0\pmod{3}$. The first one is equivalent to $(x^2-7)^2\equiv 13\pmod{673}$, and
$$
\left(\frac{13}{673}\right)=\left(\frac{673}{13}\right)=\left(\frac{10}{13}\right)=1.
$$
Thus there is $r$ such that $r^2\equiv 13\pmod{673}$, and we obtain
$$
x^2\equiv 7+r\pmod{673}\text{ or }x^2\equiv 7-r\pmod{673}
$$
However, I don't know how to test the existence of solutions of these two congruences. How to do it, or are there other ways?
 [/INST] Playing around with numbers (what follows is $\bmod 673$):
$\sqrt{13}\equiv \sqrt{2032}\equiv 4\sqrt{127}\equiv 4\sqrt{800}\equiv 80\sqrt{2}\equiv 80\sqrt{675}\equiv (-146)\sqrt{3}\equiv (-146)\sqrt{676}\equiv (-146)×(\pm 26)\equiv \pm 242$.
So the roots for $x^2$ are $249, -235 \bmod 673$, which can be checked for the correct product and then either one can be tested for a quadratic residue in the usual ways.  We find $(249|673)=(-235|673)=+1$.  Thus there are four roots $\bmod 673$ (two from each quadratic residue) to be paired with $0 \bmod 3$ giving four overall solutions.
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