1. Define Newtons second law of motion (this will help put things into perspective)
2.Get the mass of the object (in this case 75 kg)
3.The net force acting on the object...find it (in this case 500 N)
4.Change the equation to F=ma (500=75a)
5.Divide both sides by 75 and that is the acceleration.
Answer:

Explanation:
The relative velocity can be calculated by means of the difference between vector B minus vector A.

An impulsive force is a force that is acting only during a short time, I mean, for an instant. Impulse is a physics magnitude define by the product of the impulsive force and the time that it was acting.
Is there any mistake in my English? Please, let me know.
Suppose car A is moving with a velocity Va, and car b with a velocity Vb,
According the principle of conservation of momentum:
Va x Ma + Vb x Mb = (Ma + Mb) V
V = (Va x Ma + Vb x Mb)/(Ma +Mb)
V = speed of cars after coupling
V = (Va x 20 mg + Vb x 15 mg)/(20 mg + 15 mg)
Put in the values of Va and Vb, and get the V