300km=pi(r)^2(.0006)

300   =   π · r² · 0.0006

First, divide both sides by 0.0006

500 000  =  π · r²

Now, divide both sides by pi:

159 154   =   r²

Finally, take the square root of both sides:

r   =   398.9   ≈   400

I can't help you with the units because I don't know the units of  0.0006  (and I find it strange that the 300 is in km).