From the equation and the formulas of reactants, the limiting reagent is Ag+ ie. I- in only 200mL of KI would be used in the reaction, while all Ag+ would be used up (theoretically). Since KNO3 is completely soluble in water (so it's not a solid, all NO3- ions are free), there would be 0.2*0.3 = 0.06mol of NO3- present in the final solution. And there is 200mL + 300mL = 500mL solution in total...
(Btw...haven't you already graduated from school?
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