Chemical equilibrium between gaseous reactants and products is shown.

n2(g) + 3h2(g) ⇌ 2nh3(g)

how will the reaction be affected if the pressure on the system is decreased?

The reaction is shifted to the left.

Explanation:

Le Châtelier's principle states that when there is an dynamic equilibrium, and this equilibrium is disturbed by an external factor, the equilibrium will be shifted in the direction that can cancel the effect of the external factor to reattain the equilibrium.When there is a decrease in pressure, the equilibrium will shift towards the side with more no. of moles of gas molecules of the reaction.The reactants side (left) has 4.0 moles of gases and the products side (right) has 2.0 moles of gases.So, decreasing the pressure will shift the reaction to the side with more moles of gas (left side).so, the reaction is shifted to the left.