I dont understnad y i can't do this prblem...?

Question

assuming no air resistance, how long does it take a penny to fall if it was dropped from the CN Tower (553 m)? accerleration due to gravity is 9.;8 m/s^2...

what i got...Givn: t=? d=553 m a=9.8m/s^2 v1=0
the equation should be...d=v1t+1/2at^2 rite?...and all i have to do is to move t by it self?...thats where i get stuck...

Answer 1

d = v1 t + 1/2 a t^2
=> 553 = 0 t + 1/2 (9.8) t^2
=> 4.9 t^2 = 553
=> t^2 = 113
=> t = √113 = 11 sec.