Answer:
DL/dt = 529 miles/h
Step-by-step explanation:
The radio station (point A) the point just up the radio station ( point B), and the variable position of the plane ( at specif t  point C) shape a right triangle wich hypothenuse L is:
L²  =  d² + x²
d is the constant distance between the plane and the ground
Then differentiation with respect to time on both sides of the equation
2*L*dL/dt  = 2*d* Dd/dt  + 2*x*dx/dt
But  Dd/dt  =  0
L*dL/dt  =  x*dx/dt
x =  5 miles    dx/dt  =  570 m/h     L  = √ d² + x²   L  √ (5)² + (2)²
L = √29    L  = 5.39 m
5.39 *DL/dt  =  5*570 m/h
DL/dt  =  5*570/5.39  miles/h
DL/dt  =  528.76  miles/h
DL/dt = 529 miles/h
