you are bing asked to derive a standard distribution, but i cant remember the name of the thing.
anyway, Here is a hint to get you started
Find the chance that there are 2 sixes on the first 19 rolls, and multiply by the chance of getting a 6 on the 20'th roll.
You can get the probability of 2 6's in 19 rolls by using the bonomial distribution.
The above argument generalises.
Can you finish from there?