you have to use M1V1 = M2V2 to find the "new" molarity for each of the ions used in the Ksp equation. Since Molarity changes with volume of solution, if you added 500 ml to 500ml, the molarity for both the ag and cl ions will be diff from what they were originally given as.
once you figure that out, you simply plug in the molarities into the ksp equation (in this qsp since its a precipitate problem) and then see if the qsp is smaller or larger than the ksp. If its larger, it shifts left and a precipitate is formed, if smaller, shifts right, so no precipitate.
hope that helps!
Also there is a video solution for that problem, maybe that will be better at explaining it.
