t = time
Multiply the acceleration by time and add to the height:
The equation becomes H(t) = -16 * t^2 + 90
set h(t) to 0
0 = -16*t^ +90
Subtract 90 from both sides:
-90 = -16*t^2
divide both sides by -16:
t^2 = -90 / -16
t^2 = 5.625
t = √5.625
t = 2.37 seconds, round to 2.4 seconds.