Penalty following change of possession

[PiggSkin]

Once the chains are set, they will not be reset unless a first down is gained... Usually, the coach will earn his Unsportsmanlike foul before the chains are set, so it would be first and 10...

However, in your scenario, it sounds like the chains were set before the USC was earned... So we had first and goal at the nine, then the 15 yard penalty was issued... Back up the ball, but there is no reason to move the chains... First and goal at the 24...