You first add all of the marbles together to know how many marbles are in the bag. When adding 8+11+7 you get 26, then you add it to the formula:
# of outcomes that satisfy the requirement/ total # of possible outcomes
you plug in 11 in the numerator (since it is the # of outcomes that satisfy the requirement), and 26 in the denominator (since it is the total # of possible outcomes)