To solve this problem look at the equation of the chemical reaction that takes place between the antacid which is sodium bicarbonate and citric acid:
3 NaHCO3 + H3C6H5O7 --> 3 CO2 + 3 H2O + Na3C6H5O7
We see that 3 molecules of NaHCO3 react with one molecule of citric acid to give one molecule of sodium citrate or Na3C6H5O7 in the final products.
When one tablet of NaHCO3 containing 0.0267 mole of the compound reacts with citric acid the number of moles of Na3C6H5O7 produced is one third the number of moles of NaHCO3 that reacted originally.
This is equal to 0.0267/3 = 0.0089 moles
When 0.0267 moles of NaHCO3 are dissolved we get 0.0089 moles of Na3C6H5O7.