Answer
304.1 mi/h
Work Step by Step
Let's represent this as a vector, in two directions, N/S, where N is positive, and E/W, where E is positive.
Airplane speed is 300 mi/h S, 50 mi/h E: $Speed = <-300, 50> \ mi/h$
Find the magnitude: $||Speed|| = \sqrt{(-300)^2+50^2} = 304.1 mi/h
