[sn1per9mm]

I'm glad we have never had to work with that big of a circuit.

If I'm understanding your question correctly:
7-9 and 10-12 are series, and are both series are in parallel.

So you know that:
R(7 to 9) = R7 + R8 + R9
R(10-12) = R10 + R11 + R12

Then set them up in parallel:
[1/R(7to9) + 1/R(10to12)]^-1