PHYSICS PLZ HELP?

(a)
Take g = 9.8 m/s
Take all upward quantities to be positive, and all downward quantities to be negative.

Final velocity, v = 19 m/s
Acceleration, a = -9.8 m/s²
Time taken, t = 2.1 s

v = u + at
19 = u + (-9.8)(2.1)
Initial speed, |u| = 39.58 m/s ≈ 40 m/s

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(b)
Initial velocity, u = 19 m/s
Acceleration, a = -9.8 m/s
Time taken, (4.8 - 2.1) s = 2.7 s

v = u + at
v = 19 + (-9.8)(2.7)
v = -7.46 m/s
Speed at 4.8 s after launch, |v| = |-7.46 m/s| ≈ 7.5 m/s

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OR:
Initial velocity, u = 39.58 m/s
Acceleration, a = -9.8 m/s
Time taken, 4.8 s

v = u + at
v = 39.58 + (-9.8)(4.8)
v = -7.46 m/s
Speed at 4.8 s after launch, |v| = |-7.46 m/s| ≈ 7.5 m/s

v = 19 and t = 2.1 so, using v = u + at we have:

19 = u - 2.1g

so, u = 39.58 m/s

Using again for t = 4.8 we have:

v = 39.58 - 4.8g => -7.46

i.e. velocity is 7.46 m/s downwards

Note: greatest height occurs when v = 0

so, 0 = 39.58 - gt

Hence, t = 39.58/g = 4.04 seconds

Therefore, after 4.04 seconds the rock is on it's way down.

Also, speed => |v|