A decrease in pressure would see a shift in the equilibrium to the left-hand side of the equation.
<em>According to a particular chemical principle, a system in equilibrium that has one of the constraints affecting reactions applied or removed would experience a change in the equilibrium position so as to annul the effects of the application/removal of the constraint.</em>
In this case, 2 moles of NO2 is present on the left-hand side as opposed to 1 mole of N2O4 on the right-hand side. A decrease in pressure will create more space for the formation of NO2 on the left-hand side. Thus, the equilibrium will shift a bit to this side so as to annul the effects of the decrease in pressure.
