Here given reaction is Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
Equivalent of HCl in step II = (10 -x)/(106) x (84)/(84) is because of 1 mole of NaHCO3 is formed and molecular mass of NaHCO3 is 84. So for II step which is done by using methyl orange we get 1 mole of NaHCO3