If you are trying to find the probability of completing two actions, you just find the probability of each action individually and then multiply them.
There are 5 + 3 + 2 = 10 marbles total.
The probability of selecting a blue marble is 2/10 or 1/5.
The probability of selecting a green marble is 5/10 or 1/2.
1/5 * 1/2 = 1/10
Therefore, the probability of selecting a blue marble, replacing it in the bag and then selecting a green marble is 1/10.