perimeter means the sides add up to 12.
Rectangles have two pairs of equal sides.
2(one side) + 2(another side) = 12
2 (one side + another side) = 12
(one side + another side) = 6
To draw a rectangle with a perimeter of 12 units, the two adjacent sides must add up to 6.
1 + 5
2 + 4
3 + 3
