So if the length of one side of a square field is represented by the expression 3-7x .Write an expression for the perimeter of the field expressed as a sum.

So plugging in what we are given
       3-7x
       |      |
3-7x|___|3-7x
       3-7x
To find the perimeter you add up all the sides
so 
(3-7x) + (3-7x) + (3-7x) + (3-7x) = P
then we combine like terms
3 + 3 + 3 + 3 -7x -7x -7x -7x = P
9 + 9 -14x -14x = P
- 28x + 18  = P

Hope this helps :)