Recall the surface area of a rectangular prism. The volume is SA = 2(lw + wh + lh), where SA = surface area, l = length, w = width, and h = height. Thus, we have all the necessary measurements to find the surface area.
SA = 2(lw + wh + lh) = 2(3*5 + 5*4 + 3*4) = 2(15 + 20 + 12) = 2(47) = 94 m².
Remember, if we work with area, the units are always the unit².
Another question you might ask is why surface area?
Well, we are trying to build the closed box. Thus, we have to find the amount of cardboard needed in order to make it. That is what the question is asking.