a) if im not wrong, i believe thats because in order for an object to keep at a constant velocity, it must have equal force acting backwards and forwards, if the backwards one were more, it would decelerate, (and if the forwards were more it would accelerate) hence the minimum for the car to keep going, is where forces going back and forwards are equal i.e. the minimum is where gravitional and kinetic forces are the same (as they act of different directions)
b) P= W/t OR P=E/t shows that work done=energy transfer therefore the kinetic energy transfer is proportional to work done
ngl i might be completely wrong, so soz if i am. good luck!