The formula is R1 X R2 over R1 + R2, so 36/12= 3 ohms. Try the 4 ohm or 2 ohm taps. Probably the 2 ohm will sound better.
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If I understand your question correctly you are only connecting one pair of speakers. If so, either the 4 or 8 ohm outs could sound best depending on the actual impedence curve of the speakers. In my experience, with effecient speakers where the amount of power available is not a serious issue (you've got more than enuf for loud listening) I have preferred the 8 ohm taps of my amps. You should use your ears to determine which taps to use.
If you are talking about connecting both pairs of speakers (I can't imagine why you would) you would probably want to use the 4, or more likely the 2 ohm taps.