Without replacement is the key here. The chance of getting anything is number of wanted outcomes/total outcomes.

If you're having trouble use a tree diagram, they are excellent for problems like this

On the first draw we have 3 red marbles (wanted) and 6 in total so the chance of a red marble is

On the second draw we now have2red marbles from a total of5so the chance is

Since our problem is an AND type you need to multiply your chances together: