To find the volume of the cylinder we multiply the area of the face by the height of the cylinder. Area pi*r^2 
First find area 
768in^3/3in = 256in^2 
Pi*r^2 = 256
r^2 = 256 / pi 
radius = root of 256/pi 
Hence radius = 9.027033337 in 
To check answer 
Pi*9.027033337^2 * 3in = 768in^3 
Finalized answer to 2 d.p 
Radius = 9.03 in