How high does a jet have to be to hear its radio call? I am a student pilot and I am learning about radio signals/communication.

I am studying for a test and a practice questions states:

You are flying circuits at a small airport and are flying at 1800ft ASL in the circuit (1000ft AGL) when you hear a radio call from a commercial jet at an airport 123 nautical miles to the south. Roughly how high does this jet need to be so that you can hear his radio call?

The only reference I can think of is that VHF signals can be heard by using a formula of: 
Nautical miles = 1.23 * √ height above ground level. The answer would be 10000 ft (AGL)?, but I am not sure if that is the right formula to use or how to figure it out.

Please help.
The formula you're using is correct, but don't forget your own altitude.

$$d = 1,23 × (\sqrt{h_1} + \sqrt{h_2})$$

So in your case:

$$123 = 1.23 × (\sqrt x + \sqrt{1000})$$
$$100 = \sqrt x + 31.623$$
$$\sqrt x = 68.377$$
$$x = 4675\ \mathrm{ft}$$