Using the slope concept, it is found that the shelf should have a measure of 0 degrees relative to the floor.
In this problem, the shelf and the floor are parallel, hence they have the same slope, meaning that the difference of their angles is of 0 degrees, thus, the shelf should have a measure of 0 degrees relative to the floor.
You can learn more about the slope concept at https://brainly.com/question/26342863