Answer:
76.37 mph
Step-by-step explanation:
The speed of wind is 54 miles per hour.
-Northwest direction forms a 45°. The west constituent needs to cancel out the 54mph.
-From this we notice that the west and northwest are the isosceles sides of a right triangle, we therefore use the Pythagorean Theorem to find the speed of the 3rd constituent(Hypotenuse):

Hence, they have to fly at 76.37 mph