Use the surface of the water as a reference.
Measure distance, velocity, and acceleration as positive downward.
Define
t = time, s
h = distance, m
a = 10 m/s², acceleration
u = 16.5 ft/s, initial velocity
The velocity at time t is v = u + at
When the ball reaches the bottom of the lake at t = 5.7s, its velocity is
v = (16.5 ft/s) + (10 ft/s²)*(5.7 s) = 73.5 m/s
Distance traveled at time t is h = ut + (1/2)at².
The ball reaches the bottom of the lake at 5.7 s.
Therefore the depth of the lake is
H = (16.5 ft/s)*(5.7 s) + 0.5*(10 ft/s²)*(5.7 s)² = 256.5 m
Answer:
The ball reaches the bottom of the lake with velocity 73.5 m/s
The depth of the lake is 256.5 m.