Answer:
a) Vf = 33.78m/s
b) t = 5.478s
Step-by-step explanation:
We can calculate final speed using:

where Vo = 21m/s; g = 10m/s2; ΔY = -35m
Vf = 33.78m/s
Now, having final speed, we can calculate the amount of time elapsed:
Vf = Vo - g * t where Vf = -33.78m/s; Vo = 21m/s; g = 10m/s2
t = 5.478s