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 mid point of a chord of the ellipse  $\frac{x^2}{4}+\frac{y^2}{16}=1$, The equation of the line where the chord lies is