The acceleration is a characteristic of the variable motions; that is, where the speed is not constant. The acceleration of a movement is the rate with which the speed varies, per unit of time.
When the speed varies uniformly, the acceleration of the movement is calculated by the following equation:
a = Δv/Δt = (v – v0)/(t – t0)
Δv = v – v0, is the difference between the final speed and the initial speed, in the time interval considered.
Δt = t – t0, is the time interval.
In our problem we have:
v0 = 0; v = 20 m/s; t0 = 0; t = 3.5 s
Substituting in the equation:
a = (20 – 0)/3.5 – 0)
a = 5.71 m/s^2
So the car has an acceleration of 5.71 m/s^2