Answer
See Diagram
Work Step by Step
For a graph of velocity as a function of time, acceleration can be found as the slope of the graph. For the first 2 seconds, the velocity is unchanging/has a slope of $0$. Therefore the acceleration is also $0$. For the 3rd and 4th seconds, the slope is $1$ and found by taking the change in y and dividing that by the change in x $$ a=\frac{\Delta y}{\Delta x}$$
