For the first event to occur, we do
#of blue marbles/total = 4/12 or 1/3
Since this is a situation without replacement, meaning the marble is not put back after the first pick, we must subtract 1 from the total when finding the second event's probability
So for the second event..
#of yellow marbles/ total-1 = 3/11
To find the probability of these two events occurring together, we multiply them together
1/3 x 3/11 = 3/33 = 1/11