To find the net force of the system, you can assign a negative value to the force moving to the right and then add the forces together.
The force of the cue ball moving to the left is +70 N (since we are assigning the left direction a positive value).
The force of the four ball moving to the right is -50 N.
Now, calculate the net force:
Net Force = Force (Left) + Force (Right)
Net Force = 70 N + (-50 N)
Net Force = 70 N - 50 N
Net Force = 20 N
Therefore, the net force of this system is 20 N.