Let me give you some advice when it comes to answering questions like this. LOOK AT THE UNITS. They can guide you to the correct answer. Remember that you can perform the same mathematical operation on them as you can on number. When you multiply like units together you get powers of units. So, if you multiply meters times meters you get meters squared (m^2 or m2 as you seem to want to write it). If you divide like units, then you cancel them out. So, meters divided by meters give you 1 or no units. So what have we here. We have a problem that asks you to find a mass when you are given the dimensions of a block and its density. By looking at the units you should be able to figure out what you need to do. Let us start with the density. It has units of g/m^3. So, this is the mass divided by the volume. So, in order to determine the mass of the block, we need to eliminate the volume that is we need to cancel out the units of volume. Since density is mass divided by volume that means that we must multiply by volume in order to get the mass. So, how do we get the volume? Volume is the product of the dimensions. In other words, you have already written out how it is we get the volume (2.5cm x 8.0cm x 4.0cm). So all you have to do is multiply what this gives you by the density and you have the mass of the block. (Note, the gram is actually a units of mass, not a unit of weight. So, whoever wrote this problem needs a refresher course in the difference between these two values.)