I have a question that says a rocket is launced from top of a a80 ft cliff with a initial celocity of 140 ft per second. the height of h of the orcket after t seconds is given by the euqation-16t^2+140t+80. how long after the rocket is launced will it be 20 ft from the ground. I have tried several problems like this and I still don't get it. can you help?

Question

I have a question that says a rocket is launced from top of a a80 ft cliff with a initial celocity of 140 ft per second. the height of h of the orcket after t seconds is given by the euqation-16t^2+140t+80. how long after the rocket is launced will it be 20 ft from the ground.  I have tried several problems like this and I still don't get it. can you help?

anonymous · Answer

set equation = 20 and solve for t

anonymous · Answer

so how do you do this?

anonymous · Answer

$$-16t^2+140t+80=20$$ i think there was a typo in the problem because it should be 
$$-16t^2$$ and not 
$$16t^2$$

anonymous · Answer

yes it should be -16t^2

anonymous · Answer

can you walk me through the step so that I understand it?

anonymous · Answer

$$-16t^2+140t+80=20$$
$$4t^2-35t-15=0$$ then probably quadratic formula

anonymous · Answer

They are using the sq rt and I do not understand this?

anonymous · Answer

What do you mean by "using sqrt"?

anonymous · Answer

r I see you do not understand it either so I will try somewhere else

anonymous · Answer

:)