Density is mass over volume:

In your case, mass is 48 grams and volume is 12cm^3
If you put that into the equation, you will get your density:
The height of the wood is 5 cm. The length is 2 cm and the width its 3 cm. Multiply that and you get 6 cm. The Volume formula is Length times Width times Height. 6 cm times 5 cm makes 30 cm, which is your width.
<span># of protons + # of neutrons = atomic mass</span>