Velocity of an object is defined as the ratio of displacement to the time taken. In other words,
velocity = displacement of the object / time taken for the displacement
In this case, the shopping cart (object) has a displacement of 15.6 m during its motion across the parking lot. The time taken for this displacement is 5.2 seconds. Thus, the velocity of the shopping cart can be estimated by the above mentioned relation as,
velocity of shopping cart = 15.6 m / 5.2 s = 3 m/s.
Thus, the shopping cart had a velocity of 3 m/s as it moved across the parking lot.
Another similar term, that is often confused with velocity, is speed. But, while speed is scalar, velocity is a vector quantity. However, in this case, since distance traveled and displacement are the same, velocity is the same as speed.
Hope this helps.