Dear Student,
Mass of 1 liter of solution:
1.05 g/cm3 times (1000 cm3 / L) = 1050 g/L
Mass of the two components of the solution,
Mass of HNO3 = 21 % of 1050 g = 220.5 g
Mass of H2O =79 % of 1050 g = 829.5 g
Molar mass of HNO3 = 63 g/mol
Moles of Nitric acid = 220.5 g / 63 g/mol = 3.5 mol
Molality = 3.5 mol / 0.829 kg = 4.22 m
Molarity = 3.5 mol / 1.00 L = 3.5 M
Regards