Terrestrial Microwave Tower Heights Question

I am in a Data Communications and Networking Technology class, and I have an assignment question which reads:

Using an outside source, such as the Internet or the library, determine the typical height of a terrestrial microwave tower. If the tower’s height is raised by 10 meters, how much farther will the tower be able to transmit?

I have spent a couple of hours looking for some site or discussion group which could provide this answer. Can someone help me with this, or at least point me in the right direction?

Thanks in advance!!

I am taking the same class online and after searching for several days found
this formula d=7.14 times the sqrt of (1.33*height of tower).
d is measured in Km.
for a 100m tower it would be
sqrt(1.33*100)*7.14, sqrt(133)*7.14, 11.53*7.14 = 82.32Km.

My problem is I forgot that I need to find the average height of a tower
so I can add 10m to it.