This is a problem dealing with probability. First of all, you pick a marble, at random, from three marbles. It stands to reason then that the likelihood of you picking any specific marble is a 1 in 3 chance, or 1/3.
Then you flip a coin. There are only two possible outcomes here, so the chance of the coin landing on any particular side is a 1 in 2 chance, or 1/2.
You then multiply these probabilities by each other to come out with a 1/6 probability of getting any specific outcome, meaning that there are 6 possible outcomes
Now there might be some confusion to as why I multiplied the probabilities together and as to why this works. If I may, allow me to alleviate any confusion.
If one thinks about it, it makes perfect sense why it should work. If you pick the green marble the option remains that you will either flip heads or tails. Hence, via picking the green marble you are left with two possible outcomes.
This remains true for the blue and the yellow marble, picking either will still leave you with two outcomes, giving 6 outcomes all together.
Hope this helps :)