The concentration in terms of molarity is simply the number of moles solute to the amount of volume of solution in liters. With this definition, the solution to the problem is as follows:
The molar mass of KCl is 74.55 g/mol.
Molarity = 21.2 g * 1 mol KCl/74.55 g * 1/500 mL * 1000 mL/1 L
Molarity = 0.569 M KCl