Hi,
d = rt t =d/r
Times are the same.
Let r represent the speed in still air
cross multiply to solve
450(r-28) = 350(r+28)
450r - 450*28 = 350r + 350*28
100r = 800*28
r = 8*28 = 224mph is the speed of the plane in still air
checking our answer
450/252 = 350/196 = 1.79 hr