Which equation is balanced?

Al+3F2→AlF3

Na+F2→2NaF

2Al+3O2→Al2O3

2Mg+O2→2MgO

2Mg + O2 - > 2MgO

It's quite simple, really, just add up the total amount of each element on both sides. If there is a difference in the amount of atoms on either side, that equation isn't balanced.

Take the first equation, Al + 3F2 - > AlF3.

There is one Al atom on each side, so it is properly balanced, but with fluorine, there are 6 on the reactant side and only 3 on the product side. So the entire equation is not properly balanced.

If it wasn't clear, the 6 fluorine atoms on the products side came from the multiplication of the coefficient on F (3) and the subscript (2). The coefficient 3 basically means there are three molecules of F2.