there are 2 ways to work it out, if not more...
If a sample of 4 balls are taken,
we can list the order of selection if they are chosen one-by-one.
RRBG red 1st, red 2nd, blue 3rd, green 4th....
If you calculate the remaining probabilities and sum them, you will have the
probability of choosing 2 reds, 1 blue and 1 green.
Alternatively there are ways to choose 4 from 9.
There are ways to choose 2 reds
There are ways to choose one blue and only 1 way to choose the green.
Any 2 reds can go with any blue, hence we multiply those numbers of ways
to find the total number of ways we can get 2 reds, 1 blue and 1 green
Hence you divide that number by the total number of ways to choose 4 from 9.