Determine the time required for a missile to reach its maximum altitude of 21 km

1. The problem statement, all variables and given/known data
The U-2 was an American spy plane capable of flying up to 21 km high at speeds over 600km/h. Determine the time required for the missile to reach its maximum altitude of the U-2.

2. Relevant equations
d=v_i t+1/2 at^2

3. The attempt at a solution
d=21000m
a=-9.81m/s^2
v_i=0
t=?

I dont know how to get time by itself and I have tried using the quadratic formula.
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 i'm confused by this question. there is a missle but you are given no information as its speed (initial or final), or it's acceleration? something is missing here. if the only thing you know is the distance (21000m) then the best you can accomplish is an equation made up of the unknown variables. there is little more you can do without any other info. you would not use the acceleration due to gravity as there is no suggestion that anything is falling - you only indicate that a missile is headed upwards. EDIT: the best answer you can probably get is to solve the quadratic: 1/2at^2 + Vi - 2100 = 0 there use -b +/- sqrt( b^2 - 4ac) / 2a