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
[tex]40 - 4.9 {x}^{2} = 0[/tex]
now all we have to do is get x by itself
so..
[tex]4.9 {x}^{2} = 40[/tex]
then
[tex] {x}^{2} = 40 \div 4.9[/tex]
[tex] {x}^{2} = 8.1632...[/tex]
now take the square root of each side
[tex]x = 2.857...[/tex]
2.857 seconds (rounded to the third decimal)
