density is mass by volume
1 mL of nitric acid solution has a mass of 1.36 g
therefore 1000 mL has a mass of 1360 g
the molarity of solution is 2.48 mol/L
1 L of solution has 2.48 mol
the mass of 2.48 mol of HNO₃ is - 2.48 mol x 63 g/mol = 156.24 g
this means that 156.24 g of the solution consists of nitric acid
therefore percentage by mass - mass of nitric acid / total mass x 100%
concentration as percentage by mass = 156.24 g / 1360 g x 100%
percentage = 11.5%