If two events are dependent on each other you have to multiply the probability of the first event times the probability of the second event.
first count all the marbles here there are 21
you are asked about picking two green marbles there are 7 out of the 21 total.
The first event would be 7/21
The second event would be 6/20 (since you didn't replace and you took out a green one)
now multiply your two fractions together:
and you get 1/10.
