When plane is flying along the wind then we can say
[tex]V_{plane} + v_{wind} = \frac{distance}{time}[/tex]
[tex]V_{plane} + v_{wind} = \frac{1575}{3.5}[/tex]
[tex]V_{plane} + v_{wind} = 450 mph[/tex]
Now when its going against the wind the speed is given by
[tex]V_{plane} - v_{wind} = \frac{distance}{time}[/tex]
[tex]V_{plane} - v_{wind} = \frac{1575}{4.5}[/tex]
[tex]V_{plane} - v_{wind} = 350 mph[/tex]
Now by the above two equations we will have
[tex]V_{plane} = 400 mph[/tex]
[tex]v_{wind} = 50 mph[/tex]