Assuming you want to find the length of the longest line segment, you would have to first determine where the longest line segment is.
If you can't do this straight away, then it'd be good to draw a diagram as I shown below, to visualise.
From there, you apply pythagora's theorem to the dimensions of the rectangular prism to find the length of the longest line segment.
I have attached the solution in a photo, including some explanation. Hope it helps :)