Nathan has three pieces of wood the first is 9.8 feet in length the second is 7.5 feet in length and the third is 12 feet in length he uses 5.2 feet from the first piece and 6.1 feet from the second piece and does not use a third he's avoid how many feet of wood does nathan not use

9.4 Ft for the first one

7.5 Ft for the second piece

12 Ft for the Third

he uses 5.2 Ft from the first piece of wood so 9.4 - 5.2 = 4.6ft

he uses 6.1 ft from the second piece of wood, 7.5 - 6.1 = 1.4 ft

Nathan didnt use any of the 3rd piece of wood, so he still have 12 Feet of wood remaining so, 12+4.6+1.4 = 18 feet of wood left that he didnt use.