SQUARE equal in area to a given RECTANGLE

From the topic of PLANE FIGURES OF EQUAL AREA, I will demonstrate how to draw a SQUARE that is equal in area to a given RECTANGLE.
The Rectange dimensions are : Length= 100mm and width= 40mm

[@Black_berries360]

Thank you

[@PaulMaganya]

how to draw that given length of 4mm

[@sniped648]

🎉