Several weeks ago I made rectangles consist of 8 triangles arrange them by using loop 'for'. What you are trying to build isn't too far from my case. Basically it's kind of arithmetic & geometry things we need to figure out. I'll give a try to solve your quest (hopefully).
Let's give a shot:
A hexagon consist of 6 triangles. 2nd layer of perimeter is an increment from the core.
Think about this;
- each triangles of 'core' creates 3 more triangles outside to make 2nd layer
- same rule for 3rd layer but with more increment with 5 more triangles
- see? The increments was simple arithmetic, 1,3,5, ..and you can guess next increments