using one of the equations for costant acceleration
s = ut + 0.5gt^2 + 80
s = 64t - 16t^2 + 80
s = -16t^2 + 64t + 80
= -16[ t^2 - 4t) + 80
= -16 ( t - 2)^2 + 64 + 80
= -16(t - 2)^2 + 144
Time to maximum height = 2 seconds
Maximum height = 144 ft
Displacement when it hits the ground = -80 so
-80 = -16t^2 + 64t + 80
solve for t
t = 5.74 seconds