What is the mass of an object with a density if 4.0g/cm3 that displaces 3.0cm3 of water?

I am assuming you mean the density is 4.0 g/ cm3 and the displaced H20 is 3.0 cm3 (3 mls when converted to cm3). Look at the units that should tell you how to solve the problem. Your answer must in g (mass), thus you take 4.0 g/cm3 times 3.0 cm3. Units are treated like numbers, for example 3 times 1/3 equals 1 - both 3's cancel leaving only 1 time 1 which equals 1. In this case you have 4 g/cm3 times 3.0 cm3, cm3's cancel leaving only g as your unit at the end. The rest is for you to solve! Enjoy your problem. I am assuming you mean the density is 4.0 g/ cm3 and the displaced H20 is 3.0 cm3 (3 mls when converted to cm3). Look at the units that should tell you how to solve the problem. Your answer must in g (mass), thus you take 4.0 g/cm3 times 3.0 cm3. Units are treated like numbers, for example 3 times 1/3 equals 1 - both 3's cancel leaving only 1 time 1 which equals 1. In this case you have 4 g/cm3 times 3.0 cm3, cm3's cancel leaving only g as your unit at the end. The rest is for you to solve! Enjoy your problem.