If there is 1.0 liters of 0.300 M at the beginning, that means there was 0.300 moles (because 1 L times 0.300 moles/L = 0.300 mol). It says that, at equilibrium, there is 0.200 mol PCL5. That means 0.100 mol PCl5 reacted. Therefore, 0.100 mol each was created of PCl3 and Cl2.
Equilibrium constant Kc can be found with this:
[tex]K_c= \frac{concentration\ of\ the\ products}{concentration\ of\ the\ reactants}\\K_c=\frac{[PCl_3][Cl_2]}{[PCl_5]}=\frac{(0.100)(0.100)}{(0.200)}=0.0500[/tex]