Question 126872
{{{h(t)=rt - 4.9t^2}}}
I'm assuming that r is the initial speed in {{{m/s}}}. 
Also, I'm assuming that the units of 4.9 are {{{m/s^2}}}.
That's your gravity terms (1/2 g). 
{{{h(t)=2940t - 4.9t^2}}}
At time t=0 seconds, the projectile is on the ground, about to be fired. 
{{{h(0)=2940(0)-4.9(0)^2}}}
{{{h(0)=0}}}
You need to find the next time h(t)=0. 
{{{2940t-4.9t^2=0}}}
{{{2940-4.9t=0}}}Divide both sides by (t).
{{{2940=4.9t}}}Add (4.9t) to both sides
{{{t=2940/4.9}}}Divide both sides by (4.9).
{{{t=600}}}
In 600 seconds, the projectile will hit the ground. 
600 seconds = 10 minutes.