Now remember that this "t" should really be called something like "tFinal"... the times that the cannonballs are in the air are actually from time 0 up to this time "tFinal" that you found.
You will need to choose s and z values to make your plots. Good choices would be things like you saw in the earlier parts.
Make the plots for all of the {d,t} pairs and explain why some of them don't really represent solutions that work in the real world.
For example, the solution {d=0, t=0} just means that firing the first shot, waiting no time, and firing the second shot results in them both being at ground level when they start. (Make sense?)