Question

The impedance looking into nodes 1 and 2 is:

Answer 1

To find it insert a current and determine resultant voltage, then take the ration, you also insert a voltage and determine resultant current, then take ratio.

Answer 2

thanks a lot @Kenl.JW! I inserted a constant current source of 5A and found the voltage across 100 ohm resistor to be 250V. 50 ohms is the answer and it seems to be right!