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

part (a)

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.

part (b)

The above argument generalises.

Can you finish from there?