Final answer:
If reactants or products are added or removed from a chemical reaction, the reaction will shift in the direction that will restore the original balance.
Step-by-step explanation:
If H2 is added, there is now more reactant, so the reaction will shift toward products to reduce the added H2. If NH3 is added, there is now more product, so the reaction will shift toward reactants to reduce the added NH3. If NH3 is removed, there is now less product, so the reaction will shift toward products to replace the product removed.