14.

Both are ionic compounds and therefore oxidize to certain states. Mg has an oxidation state of +2, Cl has an oxidation state of -1, Na has an oxidation state of +1, and O has an oxidation state of -2.
15.

Using the same oxidation states, you write the formulas of your products.
16.

You know that you must have the same number of elemental atoms on one side of the reaction as you do on the other. With this balanced equation, you see that both the left and right sides have 1 Mg, 2 Cl, 2 Na, and 1 O.
As a bonus, you can also add in the states of the compounds:
17. This is a double replacement reaction (also called a metathesis reaction, or a double displacement reaction), because one element within one compound basically switches places with the element in the other compound. It is also a precipitation reaction, because a precipitate (MgO) is formed.
Hope this is helpful!