word prob algebra 0

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                                                                                    SOLVE THE PROBLEM BY COMPLETING THE SQUARE 

FALLING OBJECTSKeisha throws a
rock down an old well.The distance d(in
feet) the rock falls after tseconds can be
represented by the equation d =16t^2+64t.
If the water in the well is 80 feet
below ground,how many seconds will it
take for the rock to hit the water?