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Average acceleration: i have no idea how to do this.

Aball is thrown from a height of 139 feet with an initial downward velocity of 15 ft/s. the ball's height h (in feet) after t seconds is given by the following. h=139-15t-16t^2 how long after the ball is thrown does it hit the ground? round your answer(s) to the nearest hundredth. (if there is more than one answer, use the "or" button.) t =

Iwill give ! if m∠a=59 and m∠3=81°, what is m∠b? m∠b= _[blank]_°