Current flows in zener diode
The first equation is correct.
The 0.7V comes from the 2nd zener diode which is forwards biased so becomes a standard diode.
The other diode is in reverse zener breakdown (becaues Vi &gt;&gt; Vz) so the total voltage across the two diodes is Vz + 0.7.
Now that you have the voltage on both sides of the resistor, the current through it is that voltage difference divided by R.
&quot;Rz&quot;, presumably the zener resistance, isn't really a thing. Diodes are non-linear components and as such do not behave like resistors.