#### Answer

$9.9$ miles

#### Work Step by Step

$d=k\sqrt h$
$d=$ 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