Does the prism look something like this?

Part A:
In the purple writing, I found the height of the triangle using the Pythagorean theorem.
I think this is what the question is calling the "height of the box".
height of the box = 15 cm
Part B:
The amount of cardboard needed for the sides will be the lateral area of this prism.
lateral area = perimeter of triangle * height of prism
lateral area = (16 + 17 + 17) * 20
lateral area = 50 * 20
lateral area = 1,000 cm2