0 = 400 - 600t^2
transpose for t
I get t = -400/-600^2
I am not confident that this right, please advise?
No, it's not correct. You really should simplify each step before taking on any other step... E.g. $\displaystyle \displaystyle \frac{-400}{-600} = \frac{2}{3}$...
Also, to undo squaring, you don't square both sides. You take the square root of both sides.