<s>[INST] Can only the middle school math knowlegde help to find solutions for $2013 y^2 -xy -4026 x=0$? I found the following equation form an answer written for a question.
$$2013 y^2 -xy -4026 x=0$$
But I'm confused that can I really learn how to find the positive integer solutions for $x,y$ with having only the middle school mathematics knowledge. How do I find the positive integer solutions for this equation? 
 [/INST] An scholar way can be the following:
Put $a=2013=3\cdot 11\cdot 61$; we see there are $(1+1)(1+1)(1+1)=8$ possible factors. 
$ay^2-xy-2ax=0\Rightarrow \color{red}{a|xy}\qquad (1)$.
$ay^2-xy-2ax=0\Rightarrow 2ay=x\pm \sqrt{x^2+8a^2x}\Rightarrow \color{red}{x^2+8a^2x=z^2}\Rightarrow x^2+8a^2x=w^2x^2\Rightarrow \color{red}{x|8a^2}\qquad (2)$.
So $0<x|2^3\cdot3^2\cdot11^2\cdot61^2$ hence there are at most 
$(3+1)(2+1)(2+1)(2+1)=108$ possibilities for solution $x$.
(There are not infinitely many solutions!)
From $(1)$ and $(2)$ one has $xy=2013k$ and $x^2+8a^2x=\square$ (= square)
We try $k\in \{1,2\}$ 
►$k=1$
The possible solutions are $(x,y)\in \{(1,2013),(3,671),(11,183),(33,61)\}$ and the opposite $(y,x)$ with the four given possibilities. Here we examine, using $(2)$, how squares, with eventual solutions can agree
$$\begin{cases} 1+8(2013^2=\square\\9+24(2013^2=\square\\11^2+8\cdot 11(2013)^2=\square\\33^2+8\cdot 33(2013)^2=\square\\(2013)^2(1+8\cdot2013)=\square\\(671)^2(1+8\cdot 671\cdot 3^2)=\square\\(183)^2(1+8\cdot 183\cdot11^2)= \square\\(61)^2(1+8\cdot61\cdot 33^2=\square\end{cases}$$
(the reason of the factorization in four last possibilities is to simplify the calculation).
None of these eight possibilities is square; then they are discarded.
►$k=2$
The $16$ possibilities of eventual solution for $xy=4026$, like for $k=1$, give not squares so they must be discarded.
The smallest solution, in case it exists, seems to be large. Without computer I doubt 
a student with the middle school math knowledge could find out a solution.
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