Please explain the circuit (current regulator test circuit with Zener diode)
Note that the current source and R2 form a Norton source.  I think it is easier to understand the rest of the circuit if you first transform that into a Thevenin source.  The Thevenin source voltage is:

(350 mA)(22 &Omega;) = 7.7 V

So now you have a 7.7 V source, 4 V zener, 2.1 V LED, and a total of 222 &Omega;, all in series.  The two fixed voltage drops amount to 6.1 V, so that leaves 900 mV across the 222 &Omega;.  The current thru the LED and zener is therefore:

(900 mV)/(222 &Omega;) = 4.05 mA

Now you can go back to your orginal circuit and know the voltage across R1 since we just solved for the current thru it.  From there, add the voltage across the zener and LED to get the voltage across the whole D1-R1-D2 string in your original circuit.  From that you can calculate the current thru R2.  Now you know the voltages across and currents thru everything in your circuit.