[填空题]
You stand at the top of a cliff) 3-lny7nzyhu-d8d qz while your friend stans b0f ,l8:l9ex avpbx/2 a8agsds on the ground below you. You drop a ball from rest and see that it takes 2.8 s 8b2gs:9ll v , f aae8spa/x0bxfor 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 friend threw the ball? $m/s$
