Your second answer is correct, but your first answer is incorrect. Let a be the horizontal distance between the first airliner and the airport, let b be the vertical distance between the second airliner and the airport, and let c be the distance between the two airliners. Then a and b are the distances traveled by the first plane and the second plane, respectively, during the time period that each plane has traveled. Find c using csquaredequalsasquaredplusbsquared. Rates of change are introduced by differentiating both sides of the equation with respect to time t. Substitute known values into the resulting equation and solve for the desired quantity. Check that units are consistent and the answer is reasonable