1) All matter is made of atoms. Atoms are indivisible and indestructible. 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms.
There are 6 Nitrogen atoms
Density = mass / volume
Density = (45g) / (9 [volume units])
Density = 5g / [volume unit]
There was no specification on the units of volume. However, whatever it may be, just replace the square brackets with the value and your units will be correct.