Average velocity: -22 m/s
To get the projectile's average velocity you need to determine its displacement and total time of motion.
You know that the projectile moves along the x-axis from 61 m to -29 m and that it needs 4.1 s to travel this distance.
The movement of the projectile is oriented towards the negative portion of the axis. If you take the initial point of the projectile as point zero, the projectile will end its movement to the left of this point.
This means that its displacement will be negative and equal in magnitude to
This means that the average velocity will be