A projectile enters a resisting medium at x=0 with an initial velocity of vo=900ft/s and travels 4 inches before coming to rest. Assuming that the velocity of the projectile is defined by the relation v=vo-kx, where v is expressed in ft/s and x is in feet, determine (a) the inital acceleration of the projectile, (b) the time required for the projectile to penetrate 3.9 in. into the resisting medium.