Hello, Mauro!
Are you really having difficulty reasoning these out?
. . . $\displaystyle \begin{Bmatrix} RR & RG & RB \\ GR & GG & GB \\ BR & BG & BB\end{Bmatrix}$A box contains 3 marbles: 1 red, 1 green and 1 blue.
Consider an experiment that consists of taking one marble from the box,
replacing it, and then drawing a second marble from the box.
(a) Describe the sample space.
. . . $\displaystyle \begin{Bmatrix}RG & RB \\ GR & GB \\ BR & BG\end{Bmatrix}$(b) Describe the sample space when the first marble is not replaced.