Drawing a Tree diagram will help you visualise and solve problem a and b fairly easily. but for problem c you have to use binomial distribution 1 - P( x < 6 ). Hope it helps.
I guess drawing a tree diagram with 8 stages might be a little too complicated but it will greatly help to analyse and solve the problem at hand.
Anyway for problem a the probability of obtaining a head or a tail is half. Therefore the answer to part a is (0.5)^6 x (0.5)^2 hope you will be able to do part b and c.