An object is thrown in the air with an initial velocity of 64 ft/s from a height of 80 ft. The equation h(t) = –16t2 + 64t + 80 models the height of the object in feet after t seconds. About how many seconds does it take for the object to hit the ground? Round your answer to the nearest hundredth of a second.
3 seconds
5 seconds
6 seconds
1 second
So...
a = -16
b = 64
c = 80
Which means to solve it you should do -64/2*-16 right? But that's 2 seconds which isn't an available answer... I've seen alot of people answer 5 but they aren't usin the b/2a method, so,I don't understand

Question

An object is thrown in the air with an initial velocity of 64 ft/s from a height of 80 ft. The equation h(t) = –16t2 + 64t + 80 models the height of the object in feet after t seconds. About how many seconds does it take for the object to hit the ground? Round your answer to the nearest hundredth of a second. 
3 seconds 
5 seconds 
6 seconds 
1 second 
So... 
a = -16 
b = 64 
c = 80 
Which means to solve it you should do -64/2*-16 right? But that's 2 seconds which isn't an available answer... I've seen alot of people answer 5 but they aren't usin the b/2a method, so,I don't understand