Question

What is the acceleration of the ball between 0 and 3 seconds?

Answer 1

The acceleration is change in speed over time
Which is the same as the slope of the graph you got over there

(y-y0)/(x-x0)= (3-0)/(3-0) = 1m/s^2

Answer 2

1 second = 1 Meter/Second
2 seconds = 2 Meters/second
3 seconds = 3 meters/second
4-Infinity = 3 meters/second
:)