Answer: Hello there!
in this case, you have a graph of velocity vs time, then in this graph is represented the function v(t).
The acceleration is defined as the integral with respect to the time of the velocity v(t), then we could calculate the mean acceleration between two times in a next way:
this is the slope between the times t2 and t1, where t2>t1 and V(t2) is the velocity at the time t2 (the final velocity) and v(t1) is the velocity at the time t1 (the initial velocity). In this case, a is the mean acceleration between these times.