the reactants for the equation
NaCl + CaS
the products of the reaction
Na₂S + CaCl₂
this a double displacement reaction where the cations of one salt form new salts with the anion of the other salt and vice versa.
the balanced reaction therefore ;
2NaCl + CaS ---> Na₂S + CaCl₂
a0 - 2
a1 - 1
a2 - 1
a3 - 1