One face of the prism has a surface area of nine square units and another face has a surface area of 21 square units. These faces share a common edge. Three is the only factor common to 9 and 21 (other than one), which means that one face measures three units by three units and the other measures three units by seven units. The face of the prism that is identical to the face of the cube is in the shape of a square, since every face of a cube is in the shape of a square.