Assume you have 1.00 L (1000 mL) of solution.
d = m / V
m = d x V = 1.23 g/mL x 1000 mL = 1230 g of solution
0.387 mol/L x 1 L = 0.387 mol HCl
0.387 mol HCl x (36.5 g / 1 mol) = 14.1 g HCl
mass of water = 1230 g solution - 14.1 g HCl = 1216 g H2O = 1.216 kg H2O
molality = mol HCl / kg water = 0.387 mol / 1.216 kg = 0.318 mol/kg (or 0.318 molal)
