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5.66 seconds from when the rock was thrown. first construct an expression to represent the distance from the water to the rock. first account for the starting distance. d = 43 + ? now add in the distance based upon the initial velocity d = 43 + 21t + ? now add in the acceleration due to gravity d = 43 + 21t - 0.5at^2 now substitute the known values into the formula. 5 = 43 + 21t - 0.5*9.8*t^2 subtract 5 from both sides and perform the multiplication 0 = 38 + 21t - 4.9t^2 we now have a quadratic function with a=-4.9, b=21, c=38. using the quadratic formula, we can find the solutions at t = 5.66 and t=-1.37 since the negative time doesn't make sense, the correct value will be t = 5.66 seconds.