The shelf must be at right angles (90 degrees) relative to the wall.
Step-by-step explanation:
Dana concern is that the shelf must be perpendicular to the wall. This means that the angle between the wall and the Dana’s shelf must be 90 degrees.
The term “perpendicular” is sometimes used in the mathematical terminology for the angle of 90 degrees. Hence, the Dana’s shelf must be at 90 degrees relative to the wall. It is to be noted here that straight lines make an angle of 180 degrees. Since the shelf is at 90 degrees, either side of the shelf would be at 90 degrees to the wall.
