We need to find out when the height = 0
70t - .99t^2 = 0 factor
t (70 - .99t) = 0
The scond factor set to 0 will give us the time we need
70 - .99t = 0
70 = .99t
t = 70 /.99 (sec)
If you have had Calculus, the velocity function is
H'(t) = 70 - 1.98 t
The velocity at 70/.99 sec is
70 - 1.98 ( 70/.99) =
= - 70 m / s
