Prove that there are no integers $x$ and $y$ such that $3x^2=13+4y^2$.
From the equation, I know that $3x^2$ must be odd and therefore equal $2k + 1$ for some integer $k$.
But I am unsure what to do after that.
I have also worked out that $k = 2(y^2 + 3)$, but I don't know if that helps at all.
My instructor noted that I should look at whether $x^2$ is even or odd, but I am at a loss.