Answer:
29107
Explanation:
Every next 'level', a ring of balls is added.
The ring has 6 sides of n balls, but you would then count all 6 corners double, so you have to subtract 6.
So Bₙ = Bₙ₋₁ + 6n - 6
This gives you a sequence of 1, 7, 19, 37, 61, 91, 127, 169, 217, 271, 331, ...
All the way up to 29107 at 100 balls.