The perimeter of a rectangle is given by:
2 × length + 2 × width
2l + 2w
2(l+w)
We want the perimeter to be 24, so the equation is

, divide both sides by 2

We then think of combination of two numbers that sum gives 12
1 + 11
2 + 10
3 + 9
4 + 8
5 + 7
6 + 6 ⇒ This combination will give us a square, not a rectangle
7 + 5 ⇒ This repeat the other combination 5+7
So we can have rectangles with combination of length and width
1 by 11
2 by 10
3 by 9
4 by 8, and
5 by 7