V=(4/3)pir^3
find smaller
V=(4/3)pi8^3
V=(4/3)pi512
V=(2048/3)pi
twice of that
Vbig=2*(2048/3)pi
Vbig=(4096/3)pi
find radius
(4096/3)pi=(4/3)pir^3
divide both sides by pi
(4096/3)=(4/3)r^3
times both sides by 3
4096=4r^3
divide both sides by 4
1024=r^3
cube root both sides
8∛2=r
aprox
10.0794
the answer you are looking for is 8∛2 or 10.0794in or something close to that
Assuming that an exponentiation sign is missing, all you need to know is that rational exponents work like this:
![a^{\frac{b}{c}}=\sqrt[c]{a^b}](https://tex.z-dn.net/?f=a%5E%7B%5Cfrac%7Bb%7D%7Bc%7D%7D%3D%5Csqrt%5Bc%5D%7Ba%5Eb%7D)
So, you have

And similarly,
![81^{\frac{7}{4}}=\sqrt[4]{25^7}=\sqrt[4]{(3^4)^7}=3^7](https://tex.z-dn.net/?f=81%5E%7B%5Cfrac%7B7%7D%7B4%7D%7D%3D%5Csqrt%5B4%5D%7B25%5E7%7D%3D%5Csqrt%5B4%5D%7B%283%5E4%29%5E7%7D%3D3%5E7)
The value of x is -3, or x= -3