Astone is thrown vertically upward with a speed of 18.0 . (a)how fast is it moving when it reaches a height of 11.0 ? (b)how long is required to reach this height?

for part a) i did: y=11m, g=9.81m/s^2, vy0= 18 m/s, y0=0, (v^2)y=?

v^2 y= v^2 y0- 2g (y-y0)
--> v^2 y=18^2m/s-2(9.8)(11m-0m)
--> vy=square root ( 324m/s-215.6)= 10.4m/s

part b) i started out with:
y=y0 + vy0t - (1/2) gt^2
--> 11= 18t m/s-(1/2) 9.8t^2
--> -11 + 18t- 9.8t^2