Answer
$v_{ave} = 6.79~km/h$
Work Step by Step
We can find the distance the balloon has moved:
$d = \sqrt{(21.5~km)^2+(9.70~km)^2+(2.88~km)^2}$
$d = 23.76~km$
We can find the magnitude of the average velocity:
$v_{ave} = \frac{23.76~km}{3.50~h}$
$v_{ave} = 6.79~km/h$
