quo [populum atque urbem...] pacto [...servare...] potisset [...amplius Romanam]
INTERROG-N-S-ABL people-M-S-ACC and city-F-S-ACC means-N-S-ABL save-INFIN be able-3-S-IMPERF-SUBJ more Roman-F-S-ACC
by what means he might be better able to save the [Roman] people and the Roman city.