Most likely, the equation is
Acceleration over a period of time =
(change in speed during the period of time)
divided by
(length of the period of time) .
for any piece of the graph.
The slope (m) of the graph is most likely used to determine the acceleration from a velocity vs time graph.
Explanation:
Acceleration is referred as the ratio of velocity with respect to time traveled by the object. The formula to find out slope of the graph = (y2 - y1) / (x2 - x1).
To find the acceleration, calculate the slope of the graph as it will denote the acceleration. When the slope of the graph is zero, it means the acceleration of the object is zero and when it is positive, the acceleration is positive.