To find the time to reach the maximum height you can subtract the constant 6 and write the function as:
h'(t) = -16t^2 + 50t
Then factor it: h'(t) = -2t(8t-25)
Then find the two roots:
t =0 and
8t - 25 = 0 ⇒ t = 25/8 = 3.125
The vertex is in the half between 0 and 3.125, i.e. [0+3.125]/2 = 1.5625
Then the time searched is approximately 1.56 seconds.