On the left side, you have 2 nitrogen. On the right, you only have one. So put a 2 in front of the NH3. That gives you balanced nitrogen.
After that step, you have 6 hydrogen (the coefficient x the subscript) on the right, so you need to get 6 on the left. You have 2 hydrogen (subscript). 6/2 = 3, so put a coefficient of 3 in front of the H2, and you’ll have 6 hydrogen.
The reason for covering the container is to make sure that the atmosphere in the beaker is saturated with solvent vapour. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the paper.
