Use Pythagoras.

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