Answer :
First you add all marbles so you know how many there are:
6+3+8= 17
Then, to find the probability you put the number of marbles which you want to find the probability of being pulled out on top of the fraction and make it out of 17
6/17
6+3+8= 17
Then, to find the probability you put the number of marbles which you want to find the probability of being pulled out on top of the fraction and make it out of 17
6/17