Answer
$Na_3PO_4$
Work Step by Step
Because Sodium is bonded to a Phosphate ion, this is an ionic bond. Thus, the charges of the Phosphate and of the Sodium must cancel out. Sodium, which is an Alkali Metal, has a charge of +1. Phosphate, or ($PO_4)3-$, has a charge of 3-. In order for the charges to cancel out, there must be 3 Sodium ions for every one Phosphate ion. Because the metal always comes first when writing the chemical formula for an ionic bond, the Chemical formula for Sodium Phosphate is $Na_3PO_4$.
