Part (a)
See the diagram below for the net for this box. We have two rhombuses that are the two bases of the rhombic prism. The six rectangles form the lateral sides.
In geometry, a net is effectively the outer surface of a 3D object, but it has been laid flat on a plane. Think of unwrapping a present without throwing any pieces away. The net would be the wrapping laid out flat. As another example, a net of a cardboard box would be laying the box flat. So you're unfolding the box. This is mentioned in part (b).
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Part (b)
The main difference from this net compared to a net for a cardboard box is that everything is a rectangle for a cardboard box. This is because a cardboard box folds up into a rectangular prism.
So in the diagram below, you'll replace each rhombus with a rectangle. You'll make all four angles 90 degrees.
