/diy/ - Do It Yourself

Retard here. How do I do question 3a?
I get questions 3b and 3c, however, for 3a I'm stupidly lost. Supposedly my actual test will have more derivations which is why I want to get good at it.

Vz_0 = where the slope of 1/Rz hits the voltage axis of the I-V characteristic map
Rz = zener diode resistor
Vs = input
Vout = output

Deriving an equation is my biggest weakness so I'm just trying to understand how it's even done. I'm thinking that I'll have to convert that reverse bias zener diode into it's uhh, other form, with the Vz source and the Rz source, and then... KVL??? I think I can convert that drawing into a better looking one and then possibly KVL it to get V_out.