To find the length of string b, you have to multiply 35 by 5. You get 175, and so when it asks how much total string you need for each bottle you must add 175 and 35, which gets you 210. Then, to find how much you need in total for 17 bottles you need to multiply 210 by 17. This gets you 3570 cm, which you can convert to 35.7m.
