Organic Chemistry-Kp value

Suppose, you have 35 mg of benzoic acid dissolved in 7 ml of dichloromethane. The resulting organic soln is then extracted w/ 12 ml of water. the dichloromethane layer is now 31 mg of benzoic acid.
What is the partition Coefficient (Kp) of benzoic acid in dichloromethane/water?

I think the equation is…