volume of rectangle-Lx2Lx6. Lenght of a side of a square is 6xA^2=864 or A^2=144 so A is 12. The volume of the cube is 1728. Then 12xL^2=12^3 or L=12 so the lenght of the rectangle is 24.
good work, the only thing is that a cube is not necessarily considered a square. A rectangular solid in which all edges are of equal length is a cube. However, it doesn't have to have 90 degree angles and thus doesn't have to be a square. Great solution nevertheless