6+3+5+7= 21 balls total
Choose 2 balls
21C2= 210 total number of ways you could have chosen the two balls
(nCr- n=total, r=choose)
Total green balls= 3
You choose first green ball out of 3
3C1
Then you choose second green ball out of the 2 green balls left
2C1
Multiply these two outcomes
3C1•2C1= 6
Probability=
6/210= 1/35