but my question is when got one more R at between the RL and the zener diode...
than how to find the two R

 

Your picture was not showing up, to be clear, this is the image you are referring to, correct?

I'm not sure of your question, but it sounds like you are asking what happens when one adds a resistor in series with the load resistor. If that's the case, add the two R values together to get an equivalent resistor, RLeq.

If you were to add the resistor in parallel with RL, you'd have an equivalent load resistance of \(R_{Leq} = \frac{1}{R_{L}^{-1} + R_{L2}^{-1}}\)


See here.