The answer would be 2 Na+ MgCl2–> 2 NaCl+ Mg
To balance out the equation you can see that the Chlorine isn’t balanced so you find a number that can be the coefficient, which is 2.
Na+ MgCl2—> (2)NaCl+Mg
Then you need to balance out the Na in the reactant part, which will also be 2.
(2)Na+ MgCl2–> (2)NaCl+ Mg
And there’s your answer