Cannot Measure Dimensions on Sloped Shape

Afternoon, folks. I'm a Sketchup noob trying to help my daughter with an assignment. I need to get the dimensions for the polygons in the attached image of the Seattle Central Library. If a polygon is perfectly horizontal or vertical, I have no trouble getting the dimensions. But as you can see in the second image, the minute I try to get the dimensions on a sloped piece I cannot get a proper read.

The selected line in the attached polygon is 96' 6". If I measure with the tape measure tool, I get the right dimensions. If I use the Dimensions tool, it gives me everything but what I need. I've tried it a million ways, and it keeps giving me dimensions drawn to puzzling places that I cannot identify or use.

I'm afraid I've screwed up my daughter's assignment by trying to be too clever. Can anybody assist?

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