We will have the following:
We remember that:

And we are given that:

Now, we will first determine the acceleration [The acceleration is shared between the car and the truck]:


Now:
a) The magnitude of the force of the car on the truck is:


So, the magnitude of the force of the car on the truck is 3000 N.
b) The magnitude of the force of the truck on the car is:


So, the magnitude of the force of the truck on the car is 1500 N.