Hi so I am trying to figure out what is going on in a diagram in the art of electronics.

Here it is below.

What I am trying to make is a similar circuit but with a 13V output, 20V input and a 6.8V zener with 120mA instead of 100mA load.

I am having trouble with the mathematics around which resistors to pick to achieve these values.

Is there any chance someone could walk me through what values to use and why for the spec above? I am lost

By the way the transistors are meant to be ZTX300's but its the principles and formulas i need help with most.