Hello, thank you for taking the time to post your question!
The underlying formula that we want to use to solve this one is
Time = Distance / Speed
For Distance we can use 9000, and for Speed we can use the Combined speed of 550 + 450 = 1000 since they are flying in directly opposite directions
Plugging in those values yields
Time = 9000 / 1000 = 9 hours, meaning that it will take 9 hours for the two planes to be 9000 miles apart
I hope that helps you get moving in a better direction on this type of question! Feel free to reach out if you have any additional questions beyond that :)
