[填空题]
A 95-kg halfback moving ayy.hpgnt; 9n5s ;m k/bt1v l l031pgyyd $4.1 \; m/s$ on an apparent breakaway for a touchdown is tackled from behind. When he was tackled by an 85-kg cornerback running at $5.5 \; m/s$ in the same direction, what was their mutual speed immediately after the tackle? $m/s$
