Ok I guess I found a "valid" solution.
I do not think this is the way the question was intended to be solved.
However, this is a correct mathematical proof that there does not exist integers that satisfy the given equation.
Consider what we found earlier:
\(x=\frac{y+\sqrt{5y^2+204}}{2}\)
The point is, consider: \(2x-y=\sqrt{5y^2+204}\) (*)
Let's proof this by contradiction:
Suppose x and y are positive integers.
Then the L.H.S is an integer.
Now, we will prove that the R.H.S is not an integer. It is in fact, an irrational number! For all integers y.
For the R.H.S to be an integer, \(\sqrt{5y^2+204}\) must be the square root of a "Perfect square"
Consider: \(y^2\) This is a perfect square for all y. As, \(\sqrt{y^2}=y\) an integer.
However, consider: \(\sqrt{5y^2}=\sqrt{5}*\sqrt{y^2}=y\sqrt{5}\) . Clearly, for all integers ,y, the \(\sqrt{5}\) will always be there.
Hence, \(\sqrt{5y^2}\) Is not an integer, it is always an irrational number.
Now, obviously, adding +204 does not make it an integer either.
Hence, \(5y^2+204\) is not a perfect square; thus, \(\sqrt{5y^2+204}\) is not an integer.
So the R.H.S of (*) is irrational, but the L.H.S was supposed to be an integer by our assumption.
This contradicts our assumption, and therefore, there does not exist integers x and y such that (*) is satisfied.
(By the way, this proof can even be extended to integers! (Need not to be only positive!))
hope this helped!