Please help.

D - Velocity is speed with direction.

Explanation:

The difference between speed and velocity is simple:

Speed can be represented as the change in distance over time, while velocity is just speed with direction.  

For example, the speed of an object that traveled 10 meters over 5 seconds traveled at 2m/s.

But, if that object was traveling, say, North, then the velocity of the object would be 2m/s North.

On a side note, any change in the velocity is just acceleration, whether it is changing direction or slowing down.

Hope that was of help :)

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s