Help with word problems in college algebra

Question

Benjamin threw a rock straight up from a cliff that was 80ft above the water. If the height of the rock h, in feet, after t seconds is given by the equation h=-16t^2+44t+80, how long will it take for the rock to hit the water?

Answer 1

Set h(t) = 0  and solve for t.

 

-16t² + 44t + 80 = 0

 

Solve this quadratic equation for t, which you should know how to do.  You will get a negative and a positive solution.  Since time starts at t=0, discard the negative solution.  The final answer is 4 sec.