In order to calculate the acceleration, let's use the following formula:
[tex]a=\frac{v_2-v_1}{\Delta t}[/tex]Where v2 is the final speed, v1 is the initial speed and Delta t is the period of time.
So, for v2 = 8, v1 = 20 and Delta t = 3, we have:
[tex]a=\frac{8-20}{3}=\frac{-12}{3}=-4\text{ m/s2}[/tex]