The speed-time graph shows how a car comes to rest in 7 seconds. The part of the graph labelled PO is a straight line. Calculate (a) the deceleration of the car between t = 2 and t = 4

At $t_1 = 2$ s, the speed $v_1 = 8$ m/s.
At $t_2 = 4$ s, the speed $v_2 = 3$ m/s.

Acceleration $a = \frac{\text{change in speed}}{\text{change in time}}$

Deceleration is the positive value of the negative acceleration.
The acceleration is $-2.5\, \text{m/s}^2$, so the deceleration is $2.5\, \text{m/s}^2$.

The deceleration of the car between $t=2$ and $t=4$ is $\boxed{\text{}2.5\, \text{m/s}^2\text{}}$