Answer:
The new force between charges is 2 N.
Step-by-step explanation:
It is given that, a 3 C charge and a 6 C charge feel 4 N of force between them.
The force between charges is given by :

r is distance between charges
Original force is :

If the 6 C is replaced with a 3 C, new force between them is given by :

Dividing equation (1) and (2) we get :

So, the new force between charges is 2 N.