At sea, the distance to the horizon is directly proportional to the square root of the elevation of the observer. if a person who is 36 feet above the water can see 7.4 miles, find how far a person 64 feet above the water can see.

Distance is directly proportional to the square root of the elevation is important here.

The square root of 36 feet (the elevation) is 6 feet. 
Equation for direct proportion: y=kx (y is directly proportionate to x, k is the amount of proportion)

7.4 miles = 6 feet * k
7.4mi = 6feet* (1.233 * whatever the conversion rate is between feet and miles)
The same applies to 64: the square root of it will be directly proportionate to the distance.
√64 = 8

8feet*k = [thenumber_we_don't_know]miles

8*1.233 = {??}
= 9.84

The answer is 9.84.