[填空题]
A ball is thrown straights3q)ay i2u8 f d+ 0/fidjyz(hi up with a speed of 30 m/s. (only fiso:h/4 ub7ng 0- ckkgrvuh5b 8ll in numerical value in the h og8u: k 4hk7bnbs/-gvc u5r0blanks)
(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? $m/s$