Answer based on context:

The Broncos kicked off their 2012 season at home against the Pittsburgh Steelers on Sunday Night Football, in a rematch of their 2011 Wild Card playoff game. After a scoreless first quarter, the Steelers grabbed the lead, with a 21-yard field goal by placekicker Shaun Suisham. The Broncos responded, with running back Knowshon Moreno rushing for a 7-yard touchdown, but the Steelers countered, with a 4-yard touchdown pass from quarterback Ben Roethlisberger to tight end Heath Miller just before halftime. In the third quarter, the Steelers added to their lead, with a 35-yard field goal by Suisham. The Broncos grabbed the lead, when quarterback Peyton Manning threw his 400th career touchdown pass&#8212;a 71-yarder to wide receiver Demaryius Thomas, but the Steelers responded early in the fourth quarter, when Roethlisberger threw a 3-yard touchdown pass to wide receiver Mike Wallace (with a failed two-point conversion attempt). With 9:23 remaining in the fourth quarter, the Broncos claimed the lead for good, when Manning threw a 1-yard touchdown pass to tight end Jacob Tamme (coupled with running back Willis McGahee receiving a pass from Manning for a two-point conversion), then added to their lead with a 26-yard field goal by placekicker Matt Prater, followed by cornerback Tracy Porter returning an interception off Roethlisberger 43 yards for a touchdown (with a failed two-point conversion attempt).

How many yards was the longest scoring play of the game?
71