Molarity is mol/L so in order to convert 68.0 g of NaCl to mol you have to use a conversion with the molar mass of NaCl:
68.0 g NaCl x 1 mol/58.5 g NaCl = 1.16 mol NaCl
Then plug 1.16 mol and 4.50 L in to the molarity equation:
M = 1.16 mol/4.50 L = 0.258 M
