What is the molarity of 750 mL of NaCl solution that contains 21.04 g of NaCl?

Molar mass of NaCl = (22.99 + 35.45) g/mol = 58.44 g/mol
No. of moles of NaCl = (21.04 g) / (58.44 g) = 0.360 mol

Molarity = (0.360 mol) / (750/1000 L) = 0.480 M

Basic stoichiometry using the unit-factor method.....

Molarity is a way of expressing concentration in moles of solute per liter of solution.

21.04g NaCl x (1 mol NaCl / 58.45g NaCl) / 0.750 L = 0.480 mol NaCl / L .... or .... 0.480 M

The precision of the answer can be expressed to either two or three significant digits because of the ambiguity of "750 mL". Ideally, a trailing decimal would be used to ensure that it is expressed to three significant digits: 750. mL