Oil spilled from a ruptured tanker spreads in a circle whose area increases at a constant rate of 6.5 {\rm mi}^2{\rm /hr}. How rapidly is radius of the spill increasing when the area is 10 {\rm mi}^2?

Question

Oil spilled from a ruptured tanker spreads in a circle whose area increases at a constant rate of 6.5 {\rm mi}^2{\rm /hr}. How rapidly is radius of the spill increasing when the area is 10 {\rm mi}^2?
The radius is increasing at