Answer:
1. Adding hydrogen gas, b. shift to the right
2. Adding a catalyst, c. No effect
3. Decreasing the pressure, a. shift to the left
Step-by-step explanation:
Hydrogen gas can be rewritten as H2. Whenever you add something to an equilibrium expression, it will shift to whichever side does not have this. So, since the reactant side has 3 moles of H2, adding more H2 to the reaction will shift to the products side, since there is no H2 there.
Adding a catalyst has no effect on equilibrium reactions.
When decreasing the pressure, equilibrium will shift to the side with the greater number of moles of gas. In this case, there are 4 moles of gas on the left, and 2 on the right, so it would shift to the left.