In 7.8 years, the planet Zolton moves halfway around its orbit, a circle of radius 3.90 1011 m centered on Helioz, its sun.
(a) What is the average speed in this interval?
(b) What is the magnitude of the average velocity for this interval?
km/s
C = 2πr
so, you have half that.
speed = distance/time
avg velocity is just the diameter/time (why?)