I understand your concept, but to help your daughter succeed in the subject, just explain to her that the width of the bookcase is a practical reference for carpentry. For the geometry of it, the bookcase can be turned on its side to plan a technical measurement.
What exactly is “width” in geometry?
These are NCTM standards https://illuminations.nctm.org/Lesson.aspx?id=1845
I understand your concept, but to help your daughter succeed in the subject, just explain to her that the width of the bookcase is a practical reference for carpentry. For the geometry of it, the bookcase can be turned on its side to plan a technical measurement.