It has been estimated that 900 curies of a radioactive substance introduced at a point on the surface of the open sea would spread over an area of 40,000 KM^2 in 40 days. Assuming that the area covered by the radioactive substance is a linear function of time T and is always circular in shape, express the radius R of the contamination as a function of T .

I have been trying this for about an hour by trying to solve for R from a=pi(rt)^2 but its not working. not sure how else to get a formula for r. thanks for the help