The acceleration due to gravity is ft/sec/sec.

The initial velocity of object is ft/sec.

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Acceleration of  a object is rate of change in velocity .

Velocity is .

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Substitute in .

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The position of a object at time is .

Velocity of  a object is rate of change in position .

Position at time is  .

Apply derivative on each side with respect to .

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The object reaches maximum height when velocity is zero.

The position of ball when time is  ft.

ft.

Substitute ft.

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The initial velocity is ft/sec. 

The initial velocity is ft/sec.