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Is it 5 // ((1//2) + 3)?
So the 5 is parallel with 3, which is in series with a 1 parallel with 2?
Right or wrong?
Your equation is correct - the verbal description is a bit confusing.
For the Thevenin equivalent source resistance, replace the voltage source(s) with shorts and current source(s) with opens and analyze the resistance looking into the output.
I think it depends on to what a and b are connected. If they are left floating, then no current runs to a, which means 5 and 3 are in series, and parallel to 1.
simulate this circuit – Schematic created using CircuitLab
Required, but never shown