[填空题]
A ball is thrown straight up with a speed of 30 m/s:bx ec2;:h b6wnkl sn1r 77nh,k y ,innm drk,k3fo-u) wtkg9(. (only fill in numerical value in the blhkk,rtmk)w ko3f,- grnd, n7n y7u (9ianks)
(a.) How long does it take the ball to reach the maximum height? $s$
(b.) What is the maximum height reached by the ball? $m$
(c.) What is its speed after 4.2 s? $m/s$