[填空题]
You stand at the top of(6 2unq(3 6vx vl:lgv*yu yeii a cliff while your friend stands on the ground below yv91bh6yk:vb z ou. You drop a ball from rest and see that it takes 2.8 s for the ball to hit the ground below. Your friend then picks up the ball and throws it up to you, such that it just comes to rest in your hand. What is the speed with which your friendv96:bv kh1bz y threw the ball? $m/s$
