The probability of picking a certain marble is the same as the possible number of desired outcomes in comparison to the number of all possible outcomes.
In this case, picking a blue marble is the desired outcome and the total number of marbles is the number of total possible outcomes.
Can you write down this probability?
Can you set up a probability and solve it to find the total number of marbles?
Once you find the total number of marbles, be sure to subtract the original number of marbles in the bag (12).
This way, you will know how many marbles Tom has to add to the bag!