The first thing I like to do is write down the important information and what I'm trying to find
length = 3 cm
width = 6 cm
height = 4 cm
mass = 80.0
Density = ??
Then I find the formula I need
The formula for density is
Do we have what we need? We have mass, but we don't have volume.
Can we use the information to find volume? Yes - if we multiply the length, width, and height together we can get the volume.
Plug in what you know
now that you know volume in your density formula
Going back to your questions to figure out how many digits your answers needs - since the smallest amount of digits is 2 (in 3.0) your answer should have two digits