Answer:
P(red and blue) = 1/12
Explanation:
A probability is the number of desired outcomes divided by the number of total outcomes.
Probability of independent events:
If two events, A and B, are independent, the probability of both happening is the multiplication of the probabilities of each event happening, that is:

P (red and blue).
Probability of choosing a red marble, then a blue marble. The marbles are replaced, so the trials are independent.
Probability of a red marble:
3 out of 3 + 5 + 4 = 12. So

Probability of a blue marble:
4 out of 12, so:

P (red and blue).

So
P(red and blue) = 1/12