well 3+4=7 so you have two different drawings which removes one marble if you draw and then you have 6. so first, you have 3/7 green and 4/7 to draw black marble. After then, you have either a 3/7 green and 3/7 black chance or a 2/7 green and 4/7 black chance to pull a color. you're more likely to pull a black marble first, then after getting a black marble you have a 50/50 chance of pulling either back or green.