Kinematics of rocket - calculate total hang time

[physicsphreak123]

For a class expierement we went outside and watched an air rocket launch into the air, and measured the total hang time.

Only knowing the time, find the velcotiy at liftoff.

the time is 7 seconds, and i understand that at the maximun height the time is 3.5 seconds. I don't know which equation to use because i am not familar with knowing only one variable.

we have 4 equations to choos from.
D = v*t
Vf = Vi + a*t
D = Vi*t + 1/2 a * t
Vf squared = Vi squared +2*a*d

Can you help me ?

 

[mathmike]

v_ = v_i - a t

0 = v_i - 9.8 * 3.5

v_i = 34.3 m/s