The rectangle is 8 metres by 5 metres.
You find the two numbers which can multiply to make 40 and see which of those also would result in a perimeter of 26. To make 40 you can have 4 and 10, 2 and 20, 1 and 40, or 8 and 5. 8 and 5 are the only lengths which give a perimeter of 26.
