A cube has 6 equal square faces. Area of one face = 56 × 56 = 3136. Total surface area = 6 × 3136 = 18816 square mm. Total surface area means the combined area of all outside faces. A cube has 6 equal square faces, so after finding one face area, multiply by 6. The answer is in square units, so the correct choice is 18816 mm².