A 28-kg rock approaches the foot of a hill with a speed of 15m/s. This hill slopes upward at a constant angel of 40 degrees about horizontal. The coefficients of static and kinetic friction between the hill and the rock are 0.75 and 0.20, respectively. 

(a) Use energy conservation to find the maximum height above the foot of the hill reached by the rock. 

(b) will the rock remain at rest at its highest point, or will it slide back down the hill? 

(c) If the rock does slide back down, find its speed when it returns to the bottom of the hill.