Protecting resistor value for zener diode in parallel with load
To find the limits of the resistor needed x, proceed as follows :

1) For zener to just turn on, Voltage across the load resistance ( 2400 ohms in your case) must be greater than or equal to the zener voltage.

Applying Voltage divider rule ; 

[ 2400 / (2400 + x) ] * Vi &gt; Vz


Vi = Input Voltage

Vz = Zener Voltage

From here, you get the maximum resistance x needed.

2) For Zener to be in its working limits, proceed as follows :

Current through the zener will be max ( Say Izmax). So 

Izmax = [ Ix - IL ] 


Ix is current through x

IL is current through Load resistance, which equals [ Vz/ RL ].

From here, you compute the current through x.

Since Ix = [ Vi - Vz ] / x ; you get the second limit for your resistance.