In-depth explanation please! Am I supposed to just add two lines? Lesson 3-6. Construct a rectangle with sides b and a.

I would answer this question as:
You can maybe duplicate both lines "A&B" to make the shape of a rectangle. So basically just draw the lines "A&B" vertically next to the horizontal examples provided to make the shape.
Hope this helped :)

No solution : there is no number of boxes that she bought