Double displacement reactions are when the cations and anions of 2 compounds are exchanged. Cation of one reactant will form a new product with the anion of the other reactant and vice versa.
MgCl₂ --> Mg²⁺ + 2Cl⁻
Na₂CO₃ --> 2Na⁺ + CO₃²⁻
the cations and anions are exchanged therefore double displacement reaction is as follows;
MgCl₂(aq) + Na₂CO₃(aq) --> MgCO₃(s) + 2NaCl(aq)