1) We need to use one single unit to express the elevation of a person's eyes.
a)
Remember that 1 foot is equal to 12 inches. And dividing 66" by 12 yields 5.5'
Now, let's plug into the formula we've been given:
b) Now, let's bear in mind that this same person has reached the top of a mountain, and now he's at 8,000 feet high:
Note that x, is always given in feet, as well as, d is in miles.