(A) f(s) = -5s + 8500
To find out when the plan will land set the f(s) = 0
(B) f(s) = -5s + 8500
f(s) = -5s + 8500
0 = -5s + 8500
-8500 = -5x
-8500 / -5 = -5x / -5
1700 = x
x = 1700
So when the altitude is at 0, which we used for f(s) = 0, it would take 1700 seconds for the plan to land or crash or explode like a bomb and the rest I will leave to your imagination....
Also, we can convert the seconds to hours. 1700 / 60 = 28.33 hr