Parabola

$3y=2x^2+16x+28$

      $=2(x^2+8x+16)-2$

$y={2\over 3}(x+4)^2-{2\over 3}$

This equation is similar to  $y=a(x-h)^2+k$  in which (h,k) is the vertex 

⇒ (m,n) = (-4, -2/3)

 $m+n=-4-{2\over 3} = -{14\over 3}$