Answer :

To calculate the braking distance traveled when a car's speed is reduced from 21 m/s to 0 m/s, you would need additional information such as the car's deceleration or braking distance coefficient. These values are necessary to determine the specific braking distance.

However, it is worth noting that the braking distance is influenced by various factors including the condition of the road, the efficiency of the brakes, and the reaction time of the driver. Therefore, a precise calculation would require more information.

In general, the braking distance can be estimated using the formula:

Braking Distance = (Initial Velocity^2) / (2 * Deceleration)

Where:
- Initial Velocity is the initial speed of the car
- Deceleration is the rate at which the car slows down

Without the specific values for the deceleration or other necessary information, it is not possible to calculate the exact braking distance.