Answer:

Explanation:
The average velocity is total displacement divided by time: 

And in the case of vertical 

where  is the total vertical displacement of the rock.
 is the total vertical displacement of the rock. 
The vertical displacement of the rock when it is thrown straight up from height  with initial velocity
 with initial velocity  is given by:
 is given by: 

The time it takes for the rock to reach maximum height is when  , and it is
, and it is 

The vertical distance it would have traveled in that time is 


This is the maximum height the rock reaches, and after it has reached this height the rock the starts moving downwards and eventually reaches the ground. The distance it would have traveled then would be:

Therefore, the total displacement throughout the rock's journey is 



Now wee need to figure out the time of the journey.
We already know that the rock reaches the maximum height at
 
and it should take the rock the same amount of time to return to the roof, and it takes another  to go from the roof of the building to the ground; therefore,
 to go from the roof of the building to the ground; therefore,

where  is the time it takes the rock to go from the roof of the building to the ground, and it is given by
 is the time it takes the rock to go from the roof of the building to the ground, and it is given by

we solve for  using the quadratic formula and take the positive value to get:
 using the quadratic formula and take the positive value to get:

Therefore the total time is 


Now the average velocity is 


