a box contains 4 red balls, 5 blue balls, and 6 green balls.
suppose that 3 balls are randomly selected.

a) what is the probability that exactly two of the selected balls are green ball?
b) what is the probability that all three of the selected balls are same color?
c) what is the probability that one ball of each color is selected?

What have you tried so far?

I tried 6/15 x 5/14 x 9/13 for a) , but seems like this is wrong

Right, you were quite correct.

The probability that you take 2 green balls when 3 are picked, is that you get them in this order:

GGN
GNG
NGG

Where G represents a green ball is picked while N represents 'not green' ball picked.

Can you now try this again and post what you get now? :)