#1 There is no area of a cube. There is only a volume.
#2 The formula is [tex]V = s^{3} [/tex] where s is the side.
Substitute 729 in V since that is the volume.
[tex]729 = s^{3} [/tex]
Take the cube root and you will get 9.
So the formula is [tex]V = s^{3} [/tex] and the side is 9.