There are 15 marbles in the bag. This gives us the probability of picking a red marble as 5/15, or 1/3. Because you replace the marble therefore making the probability the same each time, and as it's a logical 'and' operation, you multiply the probabilities like this:
(1/3)•(1/3)=(1/3)^2
Because (a/b)^c = a^c/b^c, P(two red marbles) = 1^2/3^2 = 1/9.