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I'm confused too

I think ideal voltage sources have 0 impedance (in your case resistance) so you can replace them with short-circuits

to calculate total resistance.

But resistance is always between two points.

Put an ohmmeter there

Put it where?

You have to add paralell and serial resistance for egsample 9k ohm with 9k ohm is paralell. Use thevenin.

Assuming you know nothing! Starting at the left. Imagine the 36V supply is short circuit, then you have two 24k resistors in parallel which give you 12k. This is in series with the other 12k so the total of the left components is 12k+12k=24k. In the centre we again short out the 36V supply, so we just have a 9k resistor (which is in parallel with the 24k at the left). On the right hand side the two 9k resistors in parallel give us 4.5k, which is in series with a 12k giving us a total value of 16.5k at the right. Now considering our circuit as these 3 values, we have a 24k, 9k and 16.5k in parallel. To work out the total value using 1/R=1/R1+1/R2+1/R3, then 1/R=1/24k + 1/9k + 1/16.5k 1/R=((9x16.5) + (24x16.5) + (24x9))/24 x 9 x 16.5 From this, R = 3564/760.5, So R = 4.6864k or 4686.4 ohms

If you want to check result, replace both of the 36V supplies with short circuits and connect an ohmeter (across the 9k resistor). This gives a reading of 4.69K (to 2 decilmal places). I hope this helps? By the way, stilton_ego is quite right - we assume voltage sources have zero impedance (resistance).

Any help? Or was it too late?

No it definitely helps, thanks guys!