Arrange the cubes into a 9*3 square, that is, 9 cubes long by 3 cubes wide. The surface area of the top and bottom faces is 27 sq cm each, making 54 sq cm in all. The side faces have areas 2*9*1+2*3*1=18+6=24 sq cm in total. Add this to the top and bottom areas: 54+24=78 sq cm. The picture below shows the "chocolate block" arrangement with three faces exposed (not in perspective).
