How do you add density and volume to equal mass?

You cannot add those two quantities; they are not compatible because they have different units. Think of it this way: you can't add apples and oranges to get apples (or oranges), and you can't add 2x and 3y to get 5x (or 5y), so you can't add kilograms per cubic meter and cubic meters to get kg/m3 or m3. But you CAN multiply them! If you multiply a volume of a substance by its density, you will know how much of that substance you have. For example, if you have ten gallons of water and know that water's density is 8.34 pounds per gallon, then you know you have ten times 8.34, or 83.4 pounds of water. If you have 150 milliliters of liquid mercury and know that mercury's density is 13.6 grams per milliliter, you can multiply 150 by 13.6 to get 2040 grams (2.04 kilograms). Note that you must have compatible units. If the volume is in fluid ounces but the density is in grams per cubic centimeter, you will have to convert one of them (or perhaps BOTH of them) so that they are compatible and can be multiplied.