Related Rates problem (calculus)?

Question

I'm simply stuck.


Problem:
A plane is flying horizontally 1 mile up at a speed of 500mph when it passes over a radar station.
Find the rate at which the distance from the plane to the station is increasing when the plane is 2 miles away from the station.

Answer 1

D = sqrt(1 + 500t)
dD/dt = 500/(2sqrt(1+500t))
4 = 1+500t
t = 3/500 hour
dD/dt = 250/sqrt(1+500*3/500) = 125 mph