~~an almost over simplified method is :~~

1) measure the collector voltage

2) calculate the current through the resistor with that voltage (remember that the voltage over the resistor is V_{power} - V_{c})

3) just apply Ohm's law to the transistor

4) then calculate parallel resistance with the collector resistor.

if the Collector voltage is 4.5v, the output impedance would be 9k

if the Collector voltage is 3v, its 6k

if the Collector voltage is 6v, its 12k

cheers, Iain

EDIT:

i just now see the 2nd transistor is PNP buffer, the impedance is complex here, when the integration of the output signal is positive (rising flank), the resistance is 18k, when the integration is negative (falling flank), its a few ohms.

this stage is common collector, the above method is still a handy thing to know for common emitter transistor stages tho,