The displacement is the distance that exists between the end position and initial position of an object, and is independent of the trajectory.
Then, for this problem, the object started the movement from a point at 16 meters to a point at 47 meters.
Then the displacement of the object was:

Then, the average speed of the object is the distance that displaced between the time it did (12 seconds).
So:
Where:
d is the displacement
v is the average speed
v = 2.58 m / s