Istarted the bottom question but i don’t understand how to finish

0 = -16t^2 +120t
16t^2 = 120t
16t = 120
t = 120/16

So s(t) will be at 0 twice, once in the beginning and once in the end. 

To find how long it's in the air, we need to find t when s(t) = 0

0 = -16t^2 +120t
16t^2 = 120t
16t = 120
t = 120/16    (that is how long it's in the air)