Displacement measures how far it is from the starting point and the final location, which in this case is the distance between Honolulu and Los Angeles. The difference between distance and displacement is that distance measures how far the object actually travels. For example, the airplane may not fly in a straight line and it may have detours, making the distance much greater than the displacement.
On the other hand, the plane could fly at different speeds at different periods of time. It will be so much more complicated if we have to calculate it. That's why this question only asks you to calculate the average velocity of the plane, ie.
So, we have
the required average velocity =
= 511.6 miles/hour
Hope this helps!