? : Sodium carbonate : : $\text{A}{l}_2{\left(\text{SO}\right)}_4$: Aluminium sulphate

Aluminium sulphate chemical formula is $\text{A}{l}_2{\left({\text{SO}}_{\text{4}}\right)}_{\text{3}}$. According to criss-cross method $\text{A}{l}_2{\left({\text{SO}}_{\text{4}}\right)}_{\text{3}}$ is formed from  and  ions.

By removing charges we can write the chemical formula of aluminium sulphate $\text{A}{l}_2{\left({\text{SO}}_{\text{4}}\right)}_3$ is  in the same way,

By removing charges, we can write the chemical formula of sodium carbonate is ${\text{Na}}_{\text{2}}{\text{CO}}_3$.