Joann has 3 green marbles, 5 mables, and 9 yellow marbles. what is the ratio of green to blue marbles.

3:5

That is assuming when you say "5 marbles" those 5 are blue because you said the color of the others but not for those.

The ratio is 3:5.

If you meant there are 3 green, 5 blues, and 9 yellows, then the ratio 3:5 is applicable.

However, if this isn't what you meant, then the answer is not valid, because there isn't enough information