The rock lands on the graound when h(x) = 0:
[tex]h(x)=-3(x-4)^2+79=0[/tex][tex]-3(x-4)^2=-79[/tex]Dividing both sides by -3 gives
[tex](x-4)^2=\frac{79}{3}[/tex]Taking the square root of both sides gives
[tex]x-4=\sqrt[]{\frac{79}{3}}[/tex]Finally, adding 4 to both sides gives
[tex]x=\sqrt[]{\frac{79}{3}}+4[/tex]converting to a decimal form gives
[tex]x=9.1[/tex]Hence, the rock lands on the ground after 9.1 s.
