Actually, the answer is much more simple than that:
We know that each square side is 400 m, the total area of the square is 400² = 160,000 m².
Therefore, every Area section is 160,000 m²/7 = 22,857.14 m².
Look at he upper triangle (A triangle), we know the Area of a triangle is base x height / 2, and the height of the triangle is exactly the middle of a side:
A Area = Base A (200 m) / 2
22,857.14 m² = Base A (200 m)/2
Base A = 228.57 m (first fence section)
Divide "kite" section in 2 (B1 and B2). The whole square can be divided in 2 section, going from the bottom left corner to the upper right corner, this part contains sections C, D E and B2:
Area middle square = B2 + C + D + E
160,000/2 m² = B2 Area + 22,857.14 m² + 22,857.14 m² + 22,857.14 m²
B2 Area = 11,428.57 m²
The area of B2 can be expressed as:
B2 Area = z (200 m)/2
11,428.57 m² = z (200 m)/2
Z = 114.28 m (second fence section)
The area of B1 is B Area - B2 Area:
B1 Area = 22,857.14 m² - 11,428.57 m²
B1 Area = 11,428.57 m²
The area of B1 can be expressed as:
B1 Area = x (200 m)/2
11,428.57 m² = z (200 m)/2
X = 114.28 m (third fence section)
Y = 400 m - x - 228.57 m
Y = 400 m - 114.28 m - 228.57 m
Y = 57.15 m
The side "a" from the A1 triangle is 200 m - y = 200 m - 57.15 m = 142.85 m
With Pitagoras's theorem:
c² = a² + b²
c = square root ((142.85m )² + (200 m)²)
c = 245.77 m (fourth fence section)
e = 200 - z
e = 200 - 114.28 m
e = 85.72 m
With Pitagoras's theorem:
f² = d² + e²
f = square root ((200 m)² + (85.72 m)²)
f = 217.59 m (fifth fence section)
The total is :
228.57 + 114.28 + 114.28 + 245.77 + 217.59 = 920.85 m
round to 921 m.
Same answer, different method.