First you calculate how much mole Hydrochloride there is.
n = c * V = 0.1288 * 0.03827 = 0.004929 mole.
You divide this by 2 so you get the amount of mole of the sodiumcarbonate, this is 0.00246 mole.
If you multiply this with the molecular mass of sodiumcarbonate you have the mass of your pure sodium carbonate. I think you can solve the rest
