Substitute y = 36
36 = -16t^2+80 t now solve for t
16t^2-80t + 36 = 0 use quadratic formula...you will find 2 values for t....one will be the popint it is rising (the first time it reaches 36 ft) and the other will be when it is coming back down
Quadratic formula a = 16 b = - 80 c = 36
\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)
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