How can I find the equivalent resistance of this circuit?
Norton Resistance
You already did the essential steps, e.g. remove load, replace voltage and current sources with short and open circuits respectively.
$$R_{norton}=5k\Omega||15k\Omega=3.75k\Omega$$
Norton Current
Simplify the circuit by converting current to voltage sources:



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Calculate the voltage at the load:
$$V_{a,b}=30V-\dfrac{45V}{20k\Omega}\cdot15k\Omega = -3.75V$$
Calculating the norton current:
$$I_{norton}=\dfrac{-3.75V}{3.75k\Omega}=-1mA$$
Redrawing:



simulate this circuit