The correct equation is the cube root of 729:

s=729[3]s=\sqrt{729}[3]s=729​[3]

Because the box is a cube, its volume is given by V=s3V=s^3V=s3, where sss is the length of one side in inches.
Here, V=729V=729V=729, so you reverse the process (take the cube root) to find the side length:

s3=729⇒s=729[3]s^3=729\Rightarrow s=\sqrt{729}[3]s3=729⇒s=729​[3]

That value sss tells you how many 1-inch cubes fit along one edge of the box.