Does anybody know how to solve this The area of a circular s

Does anybody know how to solve this???
The area of a circular sun spot is growing at a rate of 1,300 km2/s.
(a) How fast is the radius growing at the instant when it equals 9,000 km? HINT
(Round your answer to three decimal places.)
?????......km/s

(b) How fast is the radius growing at the instant when the sun spot has an area of 810,000 km2? HINT [Use the area formula to determine the radius at that instant.] (Round your answer to four decimal places.)
?????...km/s
Can anyone go over this step-by-step?
Thanks:)