Answer:
The slope of a position-time graph can be calculated as:

where
 is the increment in the y-variable
 is the increment in the y-variable
 is the increment in the x-variable
 is the increment in the x-variable
We can verify that the slope of this graph is actually equal to the velocity. In fact:
 corresponds to the change in position, so it is the displacement,
 corresponds to the change in position, so it is the displacement, 

 corresponds to the change in time
 corresponds to the change in time 
 , so the time interval
, so the time interval
Therefore the slope of the graph is equal to

which corresponds to the definition of velocity.