Using geometric distribution, and letting 6 throws in a row a single "event" with probably (1/6)^6 (or 6^-6, cos thats easier to type, sorry no latex)
Geometric=(1-p)^k-1 * p, where p=6^-6, and k is the number of trials required.
a) P=[1-6^(-6)]^12 * 6^(-6)
b) 1 - [1-6^(-6)]^8 * 6^(-6)
Not...