Question

If a rock falls from a height of 40 meters on Earth, the height H (in meters) after x seconds is approximately

Answer 1

Once the rock hits the ground, we know that its height will be equal to 0m
because of this, we can replace H(x) with 0
$40 - 4.9 {x}^{2} = 0$
now all we have to do is get x by itself
so..
$4.9 {x}^{2} = 40$
then
${x}^{2} = 40 \div 4.9$
${x}^{2} = 8.1632...$
now take the square root of each side
$x = 2.857...$
2.857 seconds (rounded to the third decimal)