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quadratic functions help please

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A ball is thrown into the air with an velocity of 48ft/s.its height h in feet after t seconds is given by the function h=-16t^2 + 48t +9 how many seconds does the ball reach its maximum height
h(t) = -16t2 + 48t + 9
Since the coefficient of t2 is negative (-16), we know that the parabola described by the quadratic opens downward.  Hence, the parabola's vertex will be the highest point on the parabola.  The t-coordinate of the vertex is:
t = -b/2a (in seconds)
Where b = 48 (coefficient of 48t) and a = -16 (coefficient of -16t2).  Plug in the values to get the answer.