First you calculate how long it will take for the object to fall the 90 meters. We'll take Earth's gravity acceleration as 9.8m/s²
Equation:
x = x₀ + v₀ * (t - t₀) + 1/2 * a * (t - t₀)²
x -> Final position in meters
x₀ -> Starting position
v₀ -> Starting velocity
t -> Final time
t₀ -> Starting time
a -> Acceleration
Our case:
x = 0
x₀ = 90m
v₀ = 0m/s
t = ?
t₀ = 0
a = -9.8m/s² (Minus 9.8 because it's on top of a cliff (90m) and it's falling to ground level (0m), so it's substracting)
0 = 90 - 1/2 * 9.8 * (t - 0)²
0 = 90 - 4.9t²
4.9t² = 90
t² = 90/4.9
t = √(18.36) = 4.28 seconds
To calculate the speed, just multiply the seconds it takes to hit the ground by the acceleration of Earth's gravity:
4.28 * 9.8 = 42m/s
