Internal resistance seen by the voltage source, Vx?

[Kratos321]

Hi everyone,

I have attached an image of the op-amp configuration (all op-amps are ideal). Basically, the question asks to calculate the resistance seen through the voltage source. The answer is 3 kΩ but I just don't understand how this is found. If someone could assist me, I would really appreciate it.

(By the way, I mean just resistance in the title, not internal resistance)

Cheers

 

[NascentOxygen]

You have labelled the current flowing in that top 1k resistor as I_x. Using the notation you have established, now mark in the magnitude and direction of the current in each of the other 4 input resistors.