Answer:Alright, let's solve this problem step-by-step.Given:- A square and a rectangle have the same perimeter.- The length of the rectangle is thrice its width.- The width of the rectangle is 19 cm.To find the length of one side of the square, we need to follow these steps:1. Calculate the perimeter of the rectangle. Perimeter of a rectangle = 2 × (length + width) Let the length of the rectangle be 'L' and the width be 'w'. Perimeter of the rectangle = 2 × (L + w)2. Since the square and the rectangle have the same perimeter, we can set the perimeter of the square equal to the perimeter of the rectangle. Perimeter of the square = 4 × side length 4 × side length = 2 × (L + w)3. We know that the length of the rectangle is thrice its width, so L = 3w. Substituting this in the equation, we get: 4 × side length = 2 × (3w + w) 4 × side length = 2 × 4w side length = 2w4. Substituting the given width of the rectangle (19 cm), we get: side length = 2 × 19 cm = 38 cmTherefore, the length of one side of the square is 38 cm.