To find the missing width/length for the perimeter of a rectangle:
You multiply the found length/width, in this case, the length, by 2. Since there are 2 of the same sides of a rectangle.
6*2 = 12
Now, you have to subtract the perimeter, 26, from the product.
36 - 12 = 24.
You have to divide the difference by 2 because 24 is the 2 missing sides put together.
24/2 = 12
So, the missing width is 12 feet.
From,
Loafly