h(t) =-1.67t^2/2 + 10t
h'(t) = -1.67t + 10 = 0
t =10/1.67 seconds to reach max height = close to 6 seconds
max height = h(10/1.67) = -1.67(10/1.67)^2/2 + 10(10/1.67) meters =50(1.67)= 83.5 meters
at 8 seconds speed = h'(8) = -1.67(8) + 10= about -3.36 m/sec
displacement at 8 seconds = h(8) = -1.67(8)^2/2 + 10(8) =26.56 meters