UA 3851 then UA 2621 at Houston
Mexico City → Houston → Austin
Timetable, Houston time
08:23UA 3851 from Mexico City at the gate
20:27UA 2621 to Austin leaves the gate
12 h 04 apart on the timetable
UA 2621 leaves 12 h 04 after UA 3851 arrives on the timetable: more than half a day, so not a connection we count.