You will that your question is not worded correctly We would need the times that the plane travels in both directions to find the total displacement to find the speed.
I believe what you are trying to ask is what is the resultant speed if it has components of 450 mph East and 225 mph North.
The magnitude would be square root of (450^2 + 255^2) = 503 mph
The direction would be Tan-1 of (225/450) = 26.6 degrees North of East
