Algebra: A Combined Approach (4th Edition)

$9.9$ miles
$d=k\sqrt h$ $d=$ distance to the horizon $h=$ elevation of the observer $d=7.4; h=36$ $7.4=k\sqrt {36}$ $7.4=k(6)$ $k=\frac{37}{30}$ Distance to the horizon at an elevation of $64$ feet: $d=k\sqrt {64}$ $d=\frac{37}{30}\sqrt {64}$ $d=\frac{37}{30}(8)$ $d\approx9.9$ miles