Question 933493
The formula {{{h = -16t + 234}}} describes the {{{height}}} of the rock, {{{h}}}} in feet, {{{t}}} seconds after the rock is thrown.  

How long after the rock is thrown does it hit the ground?

Set {{{h}}} to {{{zero}}} (since when height = 0, it'll hit the ground)and
solve for {{{t}}} 

 {{{0= -16t + 234}}}

{{{16t =234}}}

{{{t=234/16}}}

{{{t=14.625}}}

so, the rock will hit the ground in {{{14.625}}} seconds