Multi-step Moles Calculation

A video walk-through of a challenging 7 mark AQA A-level Chemistry question from paper 1 (June 2018).
A student added 627 mg of hydrated sodium carbonate (Na2CO3.xH2O) to 200 cm3 of 0.250 mol/dm3 hydrochloric acid in a beaker and stirred the mixture. After the reaction was complete, the resulting solution was transferred to a volumetric flask, made up to 250 cm3 with deionised water and mixed thoroughly. Several 25.0 cm3 portions of the resulting solution were titrated with 0.150 mol/dm3 aqueous sodium hydroxide. The mean titre was 26.60 cm3 of aqueous sodium hydroxide.
Calculate the value of x in Na2CO3.xH2O