A rock falls from the top of a building 80 m high. The time it takes to fall to the ground below is

We have

Final Height = -4.91t^2 + Initial Height  .....therefore ....

0 = -4.91t^2 + 80   subtract 80 from both sides

-80 = -4.91t^2        divide both sides by -4.91

[80/4.91]= t^2                take the  positive root of both sides

√[80/4.91] = t = about  4.036 seconds