Solve the all parts of given question:
Question: A city bus begins from rest at one bus stop. It accelerates at 0.5 m/s2 for 25 s, travels at a constant velocity for 45 seconds, and then comes to a stop by braking with an acceleration of -0.75 m/s2 until it arrives at the next bus stop.
Part I: What is the distance between the two bus stops?
Part II: What is the average velocity of the bus? Note: The section of road between the two bus stops is straight.)
Don't just give me the answer; explain your steps as well.