You pushed the bus 18 feet which is your distance. Distance is the total length of your path. But your actual displacment was only 9 feet. Displacment is the distance between your starting position and your final position.
The two types of speed are: constant and changing speed. Your speed was constant while pushing the bus, your speed was the same the whole time. The average speed was 3km/h. When you called me you discribed your speed, you could have given me a better discreption by telling me your direction or velocity. Velocity is the speed and direction of an objects motion. If you changed your velocity, it's called acceleration. Acceleration is a measure of how quickly the velocity of an object changes.