Answer: ~447 meters
Explanation: Since it is a rectangular park, there are 4 right angles. By having a diagonal through the rectangle, we have formed two right triangles. We know the length and width of the park but we need to know the diagonal. We can use the Pythagorean Theorem a^2+b^2=c^2.
By substituting 400 for a and 200 for b (or 200 for a and 400 for b, it doesn’t matter), we get that 160000+40000=c^2 or 200000=c^2. We then take the sqrt of each side to arrive at c=447.2135955... or about 447 meters