If it were a closed box, there would be 6 sides. two with area (2.5)(2), two with area (2.5)(4), and two with area (2)(4) square meters. You say "in middle it's open" so I presume one of the sides is missing, but I'm not sure which. Normally, with the box sittingon its bottom, it would be the top that is missing so the area would be 2(2.5)(2)+ 2(2.5)(4)+ (2)(4) square meters.