Name the length of the sides of 3 rectangles with perimeter of 12 units whole numbers only

The length would be 4 the width would be 3 or vice versa

The perimeter of a rectangle is 2(w+l)
We can find the lengths by setting the equation equal to 12.
12=2(w+l)
12÷2=(2(w+l))÷2
6=w+l
6=1+5
6=2+4
6=3+3
These are the lengths of the sides of three rectangles with a perimeter of 12 units. 
You're probably wondering why the third one has two of the same number, because that's usually how the lengths of sides of squares are, not rectangles.
Well, there's this wonderful thing about the rules of shapes.
Squares ARE rectangles.
Because they follow the rules for a rectangle, squares are also classified as rectangles. 
So, the rectangle side lengths are as follows:
1 unit by 5 units
2 units by 4 units
3 units by 3 units