Answer
See the graph below:
Work Step by Step
We have here two stages of motion.
In the first stage the particle is moving at zero acceleration from $t=0$ s to $t=2$ s.
Noting that the acceleration from the $v-t$ graph is given by the slope.
Thus,
$$a_{x1}=\dfrac{v_{x2}-v_{x1}}{t_2-t_1}=\dfrac{2-2}{2-0}=\color{red}{\bf 0}\;\rm m/s^2$$
And,
$$a_{x2}=\dfrac{v_{x2}-v_{x1}}{t_2-t_1}=\dfrac{4-2}{4-2}=\color{red}{\bf 1}\;\rm m/s^2$$
