'Fencing' is something to do with the perimeter of a shape as the post would go around the border of the shape.
The perimeter of the rectangular field is given by:
42 + 42 + 36 + 36 = 156 feet
The post is to be arranged with 6 feet gap in between each post, so the number of posts needed would be 156 ÷ 6 = 26 posts
