When trying to figure out how long can be the longest rod you can fit if you have a rectangular box that has the measurements 8ft x 12ft x 8 ft. the length is 12 the width is 9 and the height is 8.

The longest rod that can fit in a rectangular box is 17 feet long.

Given:
Length = 12 ft
Width = 9 ft
Height = 8 ft

We need to get the diagonal of the rectangular box. The rectangular box is three dimensional. So, the formula to get it is:

a² + b² + c² = d²

a = width ; b = length ; c = height

9² + 12² + 8² =  d²
81 + 144 + 64 = d²
289 = d²
√289 = d
17 = d