How can I write the formula for sodium phosphide?

Hint:Before writing the chemical formula first we have to write the chemical symbol of elements. The chemical symbol for an element is usually two letters long. The first letter of every chemical symbol is a capital letter and the second letter is a small letter.

Complete answer:
The chemical symbol is used to represent an atom or an element. The chemical symbol of an element usually consists of one or two letters from the Latin alphabet and the first letter is always capital. There are many sources from which the symbols are derived, it includes Greek name of an element or German name etc. For example: the symbol for lead is Pb and it is derived from the Latin name of lead with is plumbum, the symbol for tungsten is W and it is derived from the German name of tungsten which is Wolfram etc.
The chemical symbol for sodium is Na and the chemical symbol for phosphorus is P. sodium phosphide is an ionic compound and the ionic compounds are neutral. This means that the sum of charges on each element must be zero.
The charge on sodium ion=$N{{a}^{+}}$
The charge on phosphide ion =${{P}^{3-}}$
To neutralise this charge 3 sodium ions are required. Hence, the symbol of sodium phosphide is $N{{a}_{3}}P$ .

Note:
Sometimes a 3-letter symbol is also assigned to a new element which is a temporary symbol for the element. For example: the chemical symbol for unniloctium was Uno which was later changed to Hs after the discovery of hassium.