Explanation:
what is the basic unit of length ? m or meter ?
a cube is like a 3-dimensional square : all side lengths are equal.
the volume of a cube =
side³
side³ = 343 m³
side = cubic root(343) = 7 m
a cube has 6 sides (look at dice). the surface of a cube is therefore 6 times the area of a side² square.
so, for side = 7 the surface area of the cube is
6 × 7² = 6 × 49 = 294 m²
and in the other direction, when we have the surface area (216 m²) :
216 m² = 6 × side²
36 = side²
side = sqrt(36) = 6 m
the volume of that cube is then
6³ = 216 m³