The weight of NaCl decomposed by  $4.9\mathrm{g}\text{ of }{\mathrm{H}}_2{\mathrm{SO}}_4$ if 6 g of  sodium hydrogen sulphate and 1.825 g of HCl, were  produced in the reaction is: 

$\underset{\mathrm{xg}}{\mathrm{NaCl}}+{\mathrm{H}}_2{\mathrm{SO}}_4⟶\underset{6\mathrm{g}}{{\mathrm{NaHSO}}_4}+\underset{1.925\mathrm{g}}{\mathrm{HCl}}$
According to law of conservation of mass "mass is neither created nor destroyed during a chemical  change" 
$\therefore$Mass of the reactants = Mass of products  
$\mathrm{x}+4.9=6+1.825$
$\text{ or }\mathrm{x}=2.925\mathrm{g}$