This a constant current circuit using zener diode. Now to find the constant current you first need to find the volatge drop across Re(emitter resistor) and voltage across zener diode(suppose if the zener is of 2v and supply i.e Vcc = 5v then drop across zener will be Vz=2v).
Now if Vz=2v then
Ve(i.e voltage drop across Re) is
Ve= 2v-0.7v=1.3v(considering Vbe=0.7v).
Now that you have Re and Ve you can find Ie(emitter current) this is the constant current that will flow through the load.
So if the load is 2.5ohms then load voltage VLoad=(2.5*Ie) and if the load is 15ohms then the load voltage VLoad=(15*Ie).
