Final answer:
The average velocity of the plane over the 9-hour flight is 16.7 km/h.
Step-by-step explanation:
To find the average velocity, we need to calculate the total displacement and divide it by the total time taken. The displacement is the difference between the initial and final positions. In this case, the plane travels at +390 km/h for 5 hours, covering a distance of 1950 km, and then at -450 km/h for 4 hours, covering a distance of -1800 km. The total displacement is 1950 km + (-1800 km) = 150 km. Therefore, the average velocity is 150 km / 9 hours = 16.7 km/h (rounded to one decimal place).