So, Bob needs 2 cups of concentrate (I'll use the variable c to shorten it) for every 3.5 cups of water (w). When he has 6 cups of c, he has three times the original recipe, and therefore we need 3w as well.
3.5x2=6+1=7. So we need 7 cups of water.
Now, 6 cups of concentrate + 7 cups of water = 13 cups of orange juice.
Bob can make 13 cups of orange juice.
Hope I helped!
