**Average Velocity of a Car**

To calculate the average velocity of a car, we need to determine the total displacement and the total time taken. Average velocity is defined as the total displacement divided by the total time taken.

**Given Information:**
- The car travels a distance on a straight line in two hours.
- The car returns to the starting point in three hours.

**Determining Displacement:**

Since the car returns to the starting point, the displacement is equal to zero. This is because displacement is a vector quantity that represents the change in position from the starting point to the final point. In this case, the car ends up at the same position as where it started, so the displacement is zero.

**Determining Total Time:**

To find the total time taken, we add the time taken for the forward journey and the time taken for the return journey.
- Forward journey time: 2 hours
- Return journey time: 3 hours
- Total time: 2 hours + 3 hours = 5 hours

**Calculating Average Velocity:**

Average velocity is given by the formula:
Average Velocity = Total Displacement / Total Time

In this case, the total displacement is zero and the total time is 5 hours. Therefore,
Average Velocity = 0 / 5 = 0 m/s

**Explanation:**

The average velocity of the car is 0 m/s. This implies that the car, on average, did not change its position over the given time interval of 5 hours. Since the car traveled a distance on a straight line and returned to its starting point, the total displacement is zero. Hence, the average velocity is zero.

It is important to note that average velocity is different from average speed. Average speed is the total distance traveled divided by the total time taken, whereas average velocity takes into account the direction of motion. In this case, the car's average speed would be non-zero as it covers a certain distance, but its average velocity is zero since its displacement is zero.