Area of a rectangle = l × b

Given, area of a rectangle remains the same if the length is increased by 7 metres and the breadth is decreased by 3 metres. If the length is decreased by 7 metres and breadth is increased by 4 metres, the area is decreased by 21 sq. metres.

⇒ (l + 7)(b – 3) = lb

⇒ 7b – 3l = 21 ------ (1)

Also, (l – 7)(b + 4) = lb - 21

⇒ 4l – 7b = 7 ------- (2)

Adding (1) and (2), we get

⇒ l = 28

Thus, 7b – 84 = 21

⇒ b = 15m