The formula f 5280m can be used to convert a measurement of f feet to an equivalent measurement of m miles. if an airplane flies at 21,120 feet above the ground, how much is that in miles?

Just to help you, that formula is saying that 1 mile is equivalent to 5280 feet, i.e. the number of feet is equal to 5280 * number of miles.

So, you know the number of feet = 21,120 and have to convert that into the equivalent number of miles:

f = 5280m => m = f / 5280

m = 21,120 / 5280 = 4 miles

4 miles