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The ball started with velocity v directed vertically up, and was decelerated with the deceleration of g = 9.81 m/s^2
(the gravity acceleration at the Earth surface).
For the goals of this problem you can accept g = 10 m/s^2.
Then the equation to stop moving up is
0 = v - gt, or v = g*t = 10*3 = 30 m/s.
Answer. The initial velocity of the ball was 30 m/s.