Question 858622
Break down the forces acting on the box.
Gravity force ({{{F[g]}}}) acts normal to the plane and down the plane.
Friction ({{{F[f]}}}) acts up the plane. 
The minimum force ({{{F[m]}}} acts down the plane.
{{{F[m]+F[g]=F[f]}}}
{{{F[m]+mgsin(15)=mu*mgcos(15)}}}
{{{F[m]+543sin(15)=0.73(543)cos(15)}}}
{{{F[m]+140.5=382.9}}}
{{{F[m]=242.4}}}{{{N}}}