The key word here is "multiplying".

[Did you spot the word over there? Yes, it's multiplying and not adding]

We multiply by a certain multiplier TWICE to get from 1 to 36 (via the intervening term "c").

Clearly "c" becomes the multiplier.

In particular,

c² = 36
c = 6 (since the multiplier has been defined to be positive)

Then,
b = 1/6 (as we work backwards, we divide the current term by 6 to get to the previous term)
a = 1/36 (similar)

The first term is 1/36. The "zero"th term is 1/216, so the nth term is 1/216 times (1/6)^n.

With 36^9 (equivalent to 6^18) being part of this sequence, obviously adding 1 to it will not make it part of this sequence.