If you know
#"density" = "mass"/"volume"#
you can just multiply the volume to the other side to get
#"Density" xx "volume" = "mass"#
and then divide the density to get volume alone on one side of the equation. This gives you
#"volume" = "mass"/"density"#
If you're given values for two of the three, just plug them into one of the formulas and solve for the unknown. You can do this to solve for mass, density, or volume.
You can use this triangle to help remember.