Alright, so the hard part here is probably setting up the proportion. Well, the first proportion is 6 cups of water to 4 cups of flour, which will look like this:

.
Now you'll have to set up the second proportion, since you're gonna use 6 cups of flour. So the second proportion will look like this:

. x is the number you'll be solving for.
Set the two proportions equal to each other and solve for x. Good luck!