A tank initially contains 200 gallons of fresh water... (calculus mixing problem)?

A tank initially contains 200 gallons of fresh water, but then a salt solution of unknown concentration x lb/gal is poured into the tank at 2 gal/min. The well-stirred mixture flows out of the tank at the same rate. After 120 minutes, the concentration of salt in the tank is 1.4 lb/gal. What is the concentration (in lb/gal) of the entering brine?