The answer in here is 20ft and here is how you can get to this answer:
If we place the tree at the origin then we can say that
the bottom-left tree is at (-3,-2)
the top-right tree is at (4,1)
so the base of the rectangle is 7
the height is 3
Taking into account all the details then
perimeter = 2(7) + 2(3) = 20
Hope this is good enough for you