A 400-gal tank initially contains 100 gal of brine containing 50 lb of salt. Brine containing 1 lb of salt per gallon enters the tank at the rate of 4 gal/min, and the well mixed brine in the tank flows out at the rate of 3 gal/min. How much salt will the tank contain after 100 minutes?

So, I get the integrating factor but then I'm lost.

I(t)=(t+100)^3

Thanks for any help.