A 90.0 kilogram box of book slides from rest down a frictionless incline from a height of 5.0 m. A constant frictional force, introduced at point A, then brings the box to rest at point B which is located 19.0 m to the right of point A. (a) what is the speed of the box just before it reaches point A. (b) What is the coefficient of kinetic friction, of the surface from A toB?