you can also use the 'pyramid' method for three quantities:
mass
______________
density * volume
If you want to know the equation for mass, cover it up mass (in your head or on paper) so the image of the equestion above becomes:
_______________
density * volume
so mass = density * volume.
Likewise if you want to know the equation for volume, cover 'volume' and the image of the equation will be:
mass
________
density
so volume = mass/density
You can use this method for any equation with 3 quantities
hope that helps
oh, just noticed it the same as robdesbois post