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A rectangular prism has 6 faces with the sides being named length (l), width (w) and height (h).

The surface area is the sum of the area of each face

SA = 2 * l * w + 2 * l * h + 2 * w * h

The problem gives you that the w = 7 and the l = 31. The surface area is given as 868. We can plug these values into the equation above, to solve for the height.

868 = 2 * 31 * 7 + 2 * 31 * h + 2 * 7 * h

868 = 434 + 62 h + 14 h

868 - 434 = (62 + 14 ) h

434 = 76 h

h = 434 / 76

h = 5.7

Please let me know if you need any clarification. I'm always happy to answer your questions.