Finding output values in a circuit with a zener diode in parallel with a resistor
The result is going to be a piecewise function.  The basic idea with this sort of a problem is to write out an equation assuming the diode is in each operating mode (cutoff, forward bias, and reverse bias), figure out where the transitions between operation regions are in terms of Vin, then write a piecewise function that selects the proper equation under the proper conditions.