The mixture is at equilibrium, so a change in volume/pressure will cause the total moles of reactants and products to change.
We can find the new pressure of the system after the volume is reduced by using the equation:
But since CO(g) and H2(g) react to form an equilibrium, given by the formula:
The increased pressure will cause more of the reactants to form the product, therefore reducing the pressure (because 3 mols reactant will form 1 mol product).
So the final pressure will be more than 1.2 atm but less than the predicted 2.4 atm.