The answer to this is letter B. 4 meters. The reason for this is based on the weights of the people who are using the see-saw. You should remember that the person on one side is 600 N while the person on the other side is 450N.
This means that without knowing where to properly sit, the heavier person will just stay on the ground without any effort. The lighter person should sit 4 meters away from the pivot point so that everything will be balanced. In order to compute, you can use this equation: 600N x 3m = 1800Nm. 1800Nm/450N =4m.