how much metal is needed to cast a cubical metal box with outer side 10 inches long if the thickness of its walls should be exactly 2 inches?

Notice the picture below

so...  a cube has all sides equal, so is just a rectangular prism with equal sides
and a rectangular prism is just 6 rectangles stacked up to each other at the edges

now, notice in the graph at the bottom, one of the rectangles with a thickness of 2inches

so... just get the volume of each, and add them up, that's how much metal is needed