Assuming your calculations are ok:
You know the current through the zener diode as well as the I4 current, so you can deduce (thanks to kirchoff's law) the current through R3.
As you know Vref, you can calculate R3 easily thanks to Ohm's law.
Correct me if I am wrong....
Hope this helps
Current trough R3 is: I=Id+I14=6mA+2mA=8mA (1st Kirchoff's law)
Voltage on R3 is: V3=Vcc-Vref=15V-5.6V=9.4V (2nd Kirchoff's law)
Now that you know voltage and current using Ohms law you can calculate resistance: R3=V3/I=1175Ω
"R1 =Vref/I14 = 2800OΩ":OK
"R2 = Vo*10V / R1 = 5000Ω ":Can you explain how did you get this? I dont think this is OK.