You have to suppose that in still air the plane flies at mph. Then against the wind it flies at mph, and with the wind it flies at mph.
Then use the formulato write down the times taken for each leg of the journey; add them together and equate the answer to 5.5 hours. I'll start you off...To solve this equation, multiply throughout by to get rid of fractions. You'll then get a quadratic equation with only one positive root. There's your answer.
Can you complete this now?