I need help

[gloomy hollow]

how do i do this

[verbal violet]

for a)

the height of the platform is the height when t=0

for b)

you can set H = 9

then you get a quadratic equation which you can solve for two values of t

so the rocket is at 9 m at those two times

so you subtract one from the other to get the total time spent above 9m