there are a bag of 7 marbles, 3 red 4 green

if u pick 2 marbles, what is the probability of choosing one green and one red (order doesnt matter)?

isn't it 3/7 x 4/6?

i kno 4 nCr 1 x 3 nCr 1 divided by 7 nCr 2 is the correct answer but how is it different from 3/7 x 4/6?