Afirecracker shoots up from a hill 145 feet high with an initial speed of 80 feet per second. using the formula h(t) = −16t^2 + vt + s, approximately how long will it take the firecracker to hit the ground?

In order to o this we need to proceed like this:

0 = -16t^2+80t+145 
Use the quadratic formula. 
After that you will get:
t = 6.4 seconds
How can you get this result? well let me explai like this: 
evaluate the function at t=13, 11, 9, 6 (please do the calculations)
 H(t) = -16t^2 + 80t + 145
 H(13) = -16*13^2 + 80*13 + 145=
H(11) = -16*11^2 + 80*11 + 145=
H(9) = -16*9^2 + 80*9 + 145=
H(6) = -16*6^2 + 80*6 + 145=
I know this is going to help you a lot