These two are easy to work out if you remember the triangle trick:
Density is mass per unit volume. It is written, for example, like this g/cm3.
Density = mass/volume
Volume = mass/density
Mass = density x volume
Any easy way to remember this is with this triangle:Speed
Speed is distance travelled over a unit of time. So km per hour for example.
Speed = distance/time
Time = distance/speed
Distance = speed x time
Again you can use this triangle to remember this: