$$x = 2 m - n y = m + n , x - y = m - 2 n$$
$x=2m-ny\text{ and }2m-ny=m+n,x-y=m-2n$
$x=\frac{y\left(y+2\right)}{3}$
$m=\frac{y^{2}+y}{3}\text{ and }n=\frac{y}{3}$
$y=3n$
$x=n\left(3n+2\right)\text{ and }m=n\left(3n+1\right)$
