First of all, it is hard to know how to do this problem if you do not understand all the terms being used.
Square= a shape with four sides of equal length that meet to form four right angles.
Congruent rectangles = rectangles that are exactly the same in size and shape.
Perimeter = in this case, it is the sum of all the sides of the rectangle
The question asks you to divide a square with sides that are 6 units long into 3 congruent rectangles.
If it is hard to picture this in your head, I recommend that you draw a picture. I drew one for you so you can see how mine looks. In the picture, you can see I cut one side of the square into 3 equal pieces. So, we divide the total side length by the number of equal divisions to get the heights of the rectangles. I got height (h)= 2 units. To find the width of the rectangles, you can merely use the length of a side of the square. So, the width(w)=6 units. So, then to get the perimeter of a rectangle you add up all the sides. Perimeter= h+h+w+w= 2+2+6+6= 16 units. I got rectangle perimeter=16 units for my final answer. Hope this helped! Comment on this answer if you still have any questions or you think I messed up. :)