This is a surface area problem. Find the area of each side. Front/back are 2(12x5), or 2x60, so 120. Left/right are 2(6x5), or 2x30, so 60. Top/bottom are 2(12x6), or 2x72, so 144. Add these 3, 120+60+144=324. The number of square inches of cardboard is 324 square inches. Peace!