Given,
The acceleration of the car, a=4.20 m/s²
The time period, t=5.00 s
The car starts from the rest, ∴ the initial velocity, u=0 m/s
According to the equations of motion, the final velocity is calculated as,
On substituting the known values in the above equation,
The average velocity at a constant acceleration is calculated as,
Therefore the average velocity is,
Therefore the average velocity of the car is 10.50 m/s