Question 1139838
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            Your equation is written INCORRECTLY.


            The correct form is   h(t) = -4.9t^2 + 19.6t + 686 ,  where  h  is measured in feet.



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The ball will hit the ground, when  h(t) = 0,  or, equivalently,


    -4.9t^2 + 19.6t + 686 = 0.


Simplify by dividing by (-4.9) to get


    t^2 - 4t - 140 = 0,


    (t-14)*(t+10) = 0.


Of the two roots,  t= 14  and  t= -10,  only positive t = 14 is meaningful, giving the


<U>ANSWER</U>.  t= 10 seconds.
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Solved.