Differentiate wrt time.
Let D be the hypoteneuse formed by the triangle we get.
You can find D by just using Pythagoras right out given x=600 and y=5000.
Remember, consistent units. That is the trick here. Be careful.
Fortunately, you're given the conversions.
Also y is constant, so dy/dt=0
You have all the knowns. Enter them in and solve for dD/dt