Question

A particle of mass m rests on a rough inclined surface at an angle Pi/6
to the horizontal. When a force of magnitude P is applied parallel to the
surface pointing up the slope, the particle is on the point of slipping down the
slope, whereas when a force of magnitude 2P is applied parallel to the surface
pointing up the slope, the particle is on the point of slipping up the slope.
Show that the coecient of friction is
sqrt(3/9)?