The surface area of a rectangular prism is the combined surface areas of all six of its sides. Because opposing sides are always the same size, the total surface can be determined by using the length, width and height of the front, top and side of the prism.
To calculate the area of the front and rear of the prism, the length of the front panel is multiplied by its height, and that number is multiplied by two. For the bottom and top surfaces, the width is multiplied by the length and then multiplied by two. For the side panels, the width is multiplied by the height and then multiplied by two. The three sums together are added together to get the total surface area.