To determine how much fencing Jamila needs to build a fence diagonally across a square lot, we first need to find the length of the diagonal of the square.
Given that 1 acre is represented as a square lot, and each side of the square is 209 feet, we can use the Pythagorean theorem to find the diagonal length \(d\) of the square. The formula for the diagonal \(d\) of a square with side length \(s\) is:
\[ d = s \sqrt{2} \]
In this case, the side length \(s\) is 209 feet. Plugging in the values:
\[ d = 209 \cdot \sqrt{2} \]
To calculate \(\sqrt{2}\):
\[ \sqrt{2} \approx 1.414 \]
Now substituting this value:
\[ d \approx 209 \cdot 1.414 \approx 295.506 \]
Rounding this to the nearest foot gives approximately 296 feet.
Thus, the amount of fencing Jamila needs is:
296 feet.