Physics : From the top of 100m tall building you throw a ball downward with a speed of 10 m/s.?

1.
Take g = 9.81 m/s²
Take all downward quantities as positive.

Initial velocity, vₒ = 10 m/s
Acceleration, a = 9.81 m/s²
Displacement, s = 100 m

a)
s = vₒt + (1/2)at²
100 = 10t + (1/2)(9.81)t²
4.905t² + 10t - 100 = 0
t = [-10 + √(10² + 4×4.905×100)] / (2×4.905)
Time taken, t = 3.6 s

b)
v = vₒ + at
v = 10 + (9.81)(3.6)
Final velocity, v = 45 m/s