Trying to understand how they got equation for Vzo
According to the picture, the 6.8 V is across the series combination of both Zener voltage and the resistor.


With out further context, I can only go ahead with the image. As per the image, the equations derived are right.    



It is the representation of Zener diode as a whole. Hence the statement, the Zener diode has 6.8 V across it. When you split Zener diode into it's equivalent of voltage and a series resistor, the voltage too gets divided.