volume of the rectangular prism = base area * height
base area = length * breadth
volume of the rectangular prism = length * breadth * height
volume of the rectangular prism = 5*8*2 = 80 inches ^3
volume of taken up by the eraser = 5.5 inches^3
volume that is not take-up by the eraser = 80-5.5 = 74.5 inches^3