If the area of a square is 25 inches squared then each side measures 5 inches

Can someone tell me if this is perpendicular? !

Mrs. avila is buying a wallpaper border to go on all of her kitchen walls. she wants to buy 5 extra feet of wallpaper border than she needs to be sure she has enough. she buys 55.5 feet of the border. the store owner selling the border uses the width of mrs. avila's kitchen to determine that the length of her kitchen must be 14.5 feet.

Which geometric principle is used to justify the construction below?