Step 2 in writing the chemical formula for a compound involving calcium and nitrogen.

Here we look at the charges on the ions.
calcium+2
nitrogen-3
Drop the signs on the charges and determine the lowest common multiple (LCM) between "2" and "3".

For Ca+2 and N-3, the LCM is

2 x 3 = 6

To determine the number of ions needed,

LCM / charge of ion = number of ions

Step 2:

Number of Ca+2 ions
Number of N-3 ions
6/2 = 3 Ca+2 ions

6/3 =2 N-3 ions