Nitrogen triiodide contains 3 atoms of iodine as the name suggests (note the word "tri"). It has a chemical formula of `NI_3` . Since it decomposes into nitrogen gas (represented as `N_2`) and iodine gas (represented as `I_2`), we cna write a basic chemical equation as:

`NI_3 -> N_2 + I_2`

However, this chemical reaction is not balanced, since the number of atoms of nitrogen and iodine are different on two sides of the equation. Let us first balance the number of nitrogen atoms. There are 2 nitrogen atoms on product side, but only one on reactant side. To balance nitrogen let us use a coefficient of 2 on left hand side. This gives us:

`2NI_3 -> N_2 + I_2`

Now, the nitrogen is balanced, but iodine is not. We have 6 atoms of iodine on reactant side, but only 2 on product side. We can balance iodine by using a coefficient of 3 for iodine gas. The well-balanced chemical equation for the given reaction is:

`2NI_3 -> N_2 + 3I_2`

Hope this helps.