molarity is the number of moles of solute in 1 L of solution.
the mass of NaCl added - 20.45 g
number of moles of NaCl - 20.45 g / 58.5 g/mol = 0.350 mol
volume of the solution is 700.0 mL
since molarity is the number of moles in 1000 mL
and if 700.0 mL contains - 0.350 mol
therefore 1000 mL contains - 0.350 mol / 700.0 x 1000 = 0.500 mol
hence molarity of solution is 0.500 M