Given that $m,n,s,t\in (0,\infty );m+n=3;\frac{m}{s}+\frac{n}{t}=1;m,n$ are Constants and $m<n$ . If the minimum value of $s+t$ is $3+2\sqrt{2}$ , point $(m,n)$ is the midpoint of a chord of the ellipse  $\frac{x^2}{4}+\frac{y^2}{16}=1$ the equation of the line where the chord lies