The left mark 3 is 2× 3 = 6m away from the midpoint.

The mark 6 on the right is 6× 2 = 12m from the midpoint.

According to lever theorem

Power× power arm = resistance× resistance arm

If each glass ball is equal in weight

Let x glass balls be placed on the right, and the gravity of each glass ball is1n.

Then 4× 6 = 12x

12x=24

x=2

You need two glass balls on the right to keep balance.

Take it with your hands. thank you