Answer:
After 4.5 seconds the ball reaches ground.
Step-by-step explanation:
We equation of motion given as y = -16t²-18t+405,
We need to find in how many seconds will the ball hit the ground,
That is we need to find time when y = 0
0 = -16t²-18t+405
16t²+18t-405 = 0
[tex]t=\frac{-18\pm \sqrt{18^2-4\times 16\times (-405)}}{2\times 16}\\\\t=\frac{-18\pm \sqrt{26244}}{32}\\\\t=\frac{-18\pm 162}{32}\\\\t=4.5s\texttt{ or }t=-5.625s[/tex]
Negative time is not possible, hence after 4.5 seconds the ball reaches ground.
