To make the board balanced, the weight on the left and the right side must be equal.
Notice that the weight shown on the left side is in kg whereas the weight shown on the right side is in g.
We need to make sure that the units are consistent (same) otherwise we would get the wrong answer.
So, let us first convert the weight on the left side from kg to g.
Recall that 1 kg = 1000 g
Note that 3 1/2 kg means 3 and a half that is basically 7/2 kg
So, 3 1/2 kg is equal to 3500 g
The weight on the right side is 2600 g
Therefore, we have to add 900 g more weight to the box on the right to make the board balance.