Because the ball is stationary, the sum of the forces acting on it must be equal to 0. We can split the ball's weight into two components: perpendicular and parallel to the slope.W(para)=2gsin60W(perp)=2gcos60The frictional force is equal to the coefficient of friction multiplied by the normal reaction force. We know this reaction force, N, must balance W(perp)N=W(perp)=2gcos60Since the frictional force (F=uN) must be equal to W(para), we can find the coefficient of friction by dividing W(para) by N, to give u=tan60