The solution is prepared by mixing water in 0.785 L of 1.262 M potassium sulfate solution to obtain a final solution with a volume of 2.00 L.
One liter of a 1 M solution of any substance contains one mole of substance in one liter of the solution. 0.785 L of 1.262 M potassium sulfate solution contains 0.785*1.262 = 0.99067 moles of potassium sulfate.
In the final solution the initial amount of potassium sulfate has been dissolved in 2 L of water. This gives a molarity of 0.99067/2 = 0.495335 moles/liter.