Answer:
7.11
Explanation:
To solve this you just have to make a rule of thirds, so you have that a cylinder has a volume of 2,700 and radius 3, and you want to find out the radius of a similar cylinder volume 6,400:

So we know that the radius of a cylinder with volume 6400, and that is similar to the 2,700 cylinder with radius 3, is 7.11