We are given the equation:

Where x is the height over the sea level, where d is in miles, and x in feet. We want to know the value of the function at x = 18 feet. Thus:

We can now simplify by dividing 18 by 2:

Now, using properties of radicals:

The answer in simplified radical form is:

Using the calculator, we can find the answer to the nearest tenth of a mile d(18)= 5.2 miles