To find the length of the edge of the square base \( l \) of the pyramid, we can use the relationship between the height, the slant height, and half the length of the edge of the base.
- The height (\( h \)) of the pyramid is given as 321 feet.
- The slant height (\( s \)) is given as 436.5 feet.
- The relationship between the height, the slant height, and half the length of the edge of the base can be expressed using the Pythagorean theorem.
The relationship can be defined as: \[ s^2 = h^2 + \left(\frac{l}{2}\right)^2 \]
Substituting the values: \[ (436.5)^2 = (321)^2 + \left(\frac{l}{2}\right)^2 \]
Calculating the squares: \[ 436.5^2 = 190,196.25 \] \[ 321^2 = 103,683 \]
Now, substituting these back into the equation: \[ 190,196.25 = 103,683 + \left(\frac{l}{2}\right)^2 \]
Rearranging the equation to solve for \( \left(\frac{l}{2}\right)^2 \): \[ \left(\frac{l}{2}\right)^2 = 190,196.25 - 103,683 \] \[ \left(\frac{l}{2}\right)^2 = 86,513.25 \]
Taking the square root: \[ \frac{l}{2} = \sqrt{86,513.25} \approx 293.7 \]
Now, multiplying by 2 to find the length \( l \): \[ l \approx 2 \times 293.7 \approx 587.4 \]
Thus, the length of the edge of the square base \( l \), rounded to the nearest tenth, is: \[ \boxed{587.4} \text{ feet} \]