Alright, so these type of word problems need to be broken down into sections you can understand.
You bought 100 feet of fencing at your local garden store to go around your garden.
Your garden is a square, with the side length being 13 feet. A square is a quadrilateral that has 4 equal sides.
If fencing goes around the perimeter of a garden (which it does, in this problem) then we need to find the perimeter of the garden - the perimeter of a square can be found by multiplying the side length given by 4. ( 13(4) or 13 + 13 + 13 + 13)
The perimeter of the garden is 52 feet.
A diagram representing the situation can be drawn as a square, filled in with little flowers, and along any side, write the number 13 once. Above the drawing, you can draw picket fencing *Google white picket fence for the basic shape* and write 100 underneath it.
To find out how much fencing is left over, we subtract 52 from 100 and get 48.
And that's where part iii comes in - you can explain all I have and then do 48 feet times $12.60
48 x 12.60 = $604.80
<em>(some really expensive fencing left over!)</em>
I hope I helped you solve this!! :)