Zeros of $317x^{2}-151xy+40y^{2}$ over $\mathbb{F}_{31}$

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Let $K:=\mathbb{F}_{31}$ and $f(x,y):=317x^{2}-151xy+40y^{2}$. I have to find out if there exists any point $(a,b)\in K^{2}$ such that $f(a,b)=0$ and $a\neq0$ or $b\neq0$.

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Write $f(x, y) = 7((x + 18 y)^2 + 5y^2)$, then show that $(-5/31)= -1$ by quadratic reciprocity.