Explanation :
It is given that, the driver accelerates from a stop sign, cruises for 20 s at a constant speed of 60 km/h, and then brakes to come to a stop 40 s after leaving the stop sign.
We know that acceleration is defined as the rate of change of velocity.
[tex]a=\dfrac{dv}{dt}[/tex]
Where
dv is the change in velocity, dv = 0 - 60 m/s = -60 m/s
dt is the change in time, dt = 40 s - 30 s = 10 s
So, [tex]a=\dfrac{-60\ m/s}{10\ s}[/tex]
[tex]a = -6\ m/s^2[/tex]
From the graph it is clear that, from 30 s to 40 s the car is decelerating. So, at every second within this time the value of acceleration will be same i.e. [tex]-6\ m/s^2[/tex].