Bob needs to mix two cups of orange juice concentrate with 3.5 cups of water to make orange juice ball has 6 cups of concentrate how much orange juice can he make

Given:
2 cups of orange juice concentrate to 3.5 cups of water
6 cups of OJ concentrate available.

This is a proportion problem.

The ratio of OJ to water is 2:3.5

a:b = c:d   where ad = bc

2:3.5 = 6:x

a = 2 ; b = 3.5 ; c = 6 ; d = x

ad = bc
2x = 3.5*6
2x = 21
2x/2 = 21/2
x = 10.50 cups of water

We need 10.50 cups of water for 6 cups of orange juice concentrate.

to check:
2:3.5 = 6:10.50
2*10.50 = 3.5*6
21 = 21